Editor plugins in Sublime Text are managed with a project called Package Control.
If you just installed Sublime Text, you will need to install Package Control first.
If you have been using Sublime Text for a while, you may be able to skip to step two.
Open Sublime Text and go to Tools > Install Package Control...
Click on that! Next you should see a dialog box like this:
Click OK. Now you have Package Control available!
Note: If you cannot find Tools > Install Package Control..., it may be installed already. Try moving on to step two!
With Sublime Text open, go to Sublime Text > Preferences > Package Control
Click on that, and select Install Package from the list of options:
Type in Elm Syntax Highlighting and select the package that comes up:
Once you have Elm Syntax Highlighting installed, restart Sublime Text.
After restarting, files like src/Main.elm should have syntax highlighting!
Note: Be sure to ask in a community forum if you are having trouble getting this set up!