KMP is a mod for v0.22 of Kerbal Space Program that adds a multiplayer game option. In a KMP game you can freely interact with other players and do all the Kerbally things you'd normally do in KSP, but with friends (or strangers) playing in the same universe, at the same time. See the FAQ for more information.
Simply copy the contents of KMP.zip to your KSP directory. Be sure you copy both the "GameData" & "saves" folders as both are required for KMP to function correctly. It is recommended that you use a fresh copy of KSP for playing KMP--as KMP is in an EARLY ALPHA state it could potentially cause major issues even for non-KMP sessions, and KMP is not compatible with most other KSP mods.
To connect to a server, select it from the favourites list and click "Connect". Clicking "Add Server" will show/hide the new server form. To add a new server, just enter the address & port (default 2076) and click "Add".
For the most part, playing with KMP is just like playing a single-player KSP game: you still build and launch ships, and you can even use time warp whenever you need/want to (see "Warping & Relative Time" below). There are several new windows added to the interface to allow you to communicate and coordinate with other players, and you will see indicators for other players on all the in-game maps of the solar system. Many features (such as reverting to launch, quick-loading, pausing, & physics-warp) are disabled during a KMP session.
[Main KMP Window] The main KMP window lists all the currently connected players with status information, and provides access to most KMP features. The buttons on the main window are: "[X]" - Hides the main KMP window and the KMP privacy lock. Click the "[-]" button to restore the main KMP window. "+"/"-" - Maximizes/minimizes the main KMP window size "Detail" - Toggles whether detailed flight status information is displayed for other players "Options" - Displays settings including keybindings, as well as the "Disconnect & Exit" button "Chat" - Toggles whether the Chat window is displayed "Viewer" - Toggles whether the Viewer window is displayed "Share Screen" - Takes a screenshot to be shared with other players "Sync" - Syncronize with the server's latest "subspace" (see "Warping & Relative Time")
[Chat] The Chat window, unsurprisingly, allows you to chat with other players on the server. Press enter or click "Send" to send a message to other players. "Wide" - Toggles wide chat display-mode "Help" - Toggles display of chat commands
WARNING FOR LINUX USERS: A bug with Unity/KSP as of 0.21.1 causes keyboard input to be picked up by your vessel even when you a typing in a text box. Be sure you are on the Map View before participating in chat to prevent accidents!
[Viewer] The Viewer allows you to view screenshots shared by other players. Just click on a player's name to see the most recent screenshot they've shared.
WARNING FOR LINUX USERS: A bug with Unity/KSP as of 0.21.1 causes issues with sending or viewing screenshots. While these features may work normally in some cases, it is recommended that you avoid using them as they often cause the game to crash without warning.
Hex-editing your game binary remedies this issue as discussed on the forums here: http://forum.kerbalspaceprogram.com/threads/24529-The-Linux-compatibility-thread!?p=705281&viewfull=1#post705281
echo "7cebc7: 00" | xxd -r - KSP.x86_64
echo "7cebcc: 00" | xxd -r - KSP.x86_64
KMP allows you to use regular "on-rails" warp on-demand, as much as you need. However, whenever you use warp this desynchronizes you from other players putting you in the future relative to them. You can still play your game normally when other players are in the past, but you won't be able to interact with them directly and they'll only see the things you're doing if the "catch up" to you in time. Other players that use warp may end up in the future relative to you, and it's not uncommon to find yourself sitting between other groups of players, some in the future and some in the past. You can sync with any player that's in the future relative to you--just click the "Sync" button under their name and you'll be jumped ahead in time to the same 'subspace' as that player. The "Sync" button at the bottom of the main KMP window always sychronizes you with the "latest" subspace available in the current game, which is where (when?) newly connected players join the game as well. Be sure that you're landed or in a stable orbit when you sync, as otherwise you may find yourself somewhere unexpected!
You're still able to see other players and what they're doing in-game when they're in a different subspace--you can chat, send screenshots, share designs, etc, but you won't be able to interact with any in-game vessels that those players control. Vessels from the past or future turn translucent so that you know that you won't be able to affect them. If the other player is in the past, KMP tries to predict where they'll be in the future and shows their ship at that location. Keep in mind, though, that since the ship is still being manipulated in the past, the predicted future location can rapidly change. If another ship has been manipulated in the future, you're effectively just watching a recording of events that have "already happened" play out.
By default and vessel you launch will be in a "Public" status, which means that other players can take control of it if you leave and they may freely dock with your ship as well. To keep a vessel to yourself, use the "Lock" window on the right-side of the screen to set the vessel to "Private" status. You can't take control of another vessel owned by another player that has marked it "Private", nor can you dock with a "Private" vessel. Keep in mind, though, that if another player is in control of your vessel they can choose to lock it to themselves--only share your ships with players you trust. Your vessel locks are tied to your username & token (see "User Authentication" below)--if you change your username or token, you will lose access to vessels that were marked "Private" with those credentials. Players can always interact physically (i.e. collide) with another vessel even when the owner is absent.
KMP uses a simple username+token system for controlling your access to a server and to your vessels. Your "token" is a file created by KMP which is stored at [KSP folder]\GameData\KMP\Plugins\PluginData\KerbalMultiPlayer\KMPPlayerToken.txt--think of this file as your password. Once you first join a server successfully, the username you are using is immediately tied to your token for that server. Other players that join that server will not be able to connect with your username as they won't have a matching token. If you want to access the same server from multiple computers with a single username, be sure to copy the KMPPlayerToken.txt file created on the first computer you use to the other(s) in order to make it possible for you to connect. If your token file is missing, a new one will be generated for you but this will of course also change your token. Your vessel locks (if any) are tied to your username & token--if either changes you will lose access to any vessels that were set to "Private" until you restore the original username and token or contact a server admin for assistance.
Most mods are not supported by KMP, although some information-only mods may work OK. If all the players that use a server can agree on a common set of mod parts to install, a server admin may assist you with enabling mod parts. Never attempt to use mod parts on a server that hasn't expressly enabled them--even though KMP will attempt to stop you from doing so, if you were to successfully launch a vessel that includes parts that other players don't have, those players may encounter significant issues.
By default only the stock parts are whitelisted in "KSP_DIR/GameData/KMP/Plugins/PluginData/KerbalMultiPlayer/KMPPartList.txt". If you are playing on a server where mods are allowed, they should provide a KMPPartList.txt along with a list of mods to install. If you are a server operator, you will need to change the "_" (underscore) in part names to a "." (dot).