ERROR

[mastino123]

[info] Amidst v4.2-beta2
[info] Current system time: 2016-05-15 18:09:40.449
[info] System.getProperty("os.name") == 'Windows 7'
[info] System.getProperty("os.version") == '6.1'
[info] System.getProperty("os.arch") == 'x86'
[info] System.getProperty("java.version") == '1.8.0_91'
[info] System.getProperty("java.vendor") == 'Oracle Corporation'
[info] System.getProperty("sun.arch.data.model") == '32'
[info] Not using OpenGL.
[info] Biome profile activated: default
[info] using '.minecraft' directory at: 'C:\Users\sonobello\AppData\Roaming.minecraft', libraries: 'C:\Users\sonobello\AppData\Roaming.minecraft\libraries'
[info] using biome profiles at: 'biome'
[info] Scanning for profiles.
[info] Successfully loaded profile list.
[info] Beginning latest version list load.
[info] Attempting to download remote version list...
[info] Successfully loaded version list. URL: https://launchermeta.mojang.com/mc/game/version_manifest.json
[info] using minecraft launcher profile 'OptiFine' with versionId '1.9.4-OptiFine_HD_U_B4'
[info] Loading libraries.
[info] Found library: C:\Users\sonobello\AppData\Roaming.minecraft\libraries\optifine\OptiFine\1.9.4_HD_U_B4\OptiFine-1.9.4_HD_U_B4.jar
[info] Found library: C:\Users\sonobello\AppData\Roaming.minecraft\libraries\net\minecraft\launchwrapper\1.7\launchwrapper-1.7.jar
[crash] Amidst has encounted an uncaught exception on the thread Thread[Thread-10,1,main]
[crash] java.lang.NoClassDefFoundError: io/netty/buffer/ByteBufOutputStream
at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Unknown Source)
at java.lang.Class.getDeclaredFields(Unknown Source)
at amidst.mojangapi.minecraftinterface.RecognisedVersion.getMainClassFields(RecognisedVersion.java:132)
at amidst.mojangapi.minecraftinterface.RecognisedVersion.generateMagicString(RecognisedVersion.java:124)
at amidst.mojangapi.minecraftinterface.RecognisedVersion.from(RecognisedVersion.java:119)
at amidst.mojangapi.minecraftinterface.local.LocalMinecraftInterfaceBuilder.create(LocalMinecraftInterfaceBuilder.java:32)
at amidst.mojangapi.file.directory.VersionDirectory.createLocalMinecraftInterface(VersionDirectory.java:99)
at amidst.mojangapi.MojangApi.set(MojangApi.java:71)
at amidst.gui.profileselect.LocalProfileComponent.tryLoad(LocalProfileComponent.java:84)
at amidst.threading.WorkerExecutor.lambda$run$11(WorkerExecutor.java:71)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: io.netty.buffer.ByteBufOutputStream
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 14 more

[Treer]

As a general rule, modded profiles can't be supported.

If you don't already have a non-Optifine profile then create a new profile with the Minecraft launcher. Call it something like "Amidst" and just select that one when Amidst asks you for a profile. Use your normal optifine profile when playing Minecraft.

Optifine might be supported one day, see #110

Mod support is also touched on in the FAQ: Can I use Amidst with a modded Minecraft version?