Tools > Command Palette...
Package Control: Add Repository (until I get it in the official repository)
Add this repo (https://github.com/geetarista/ST2-Arduino)
Tools > Command Palette...
Package Control: Install Package
Enter Arduino and press enter
Download and extract to Sublime Text 2 Packages folder
Download a zipball or tarball
Extract the plugin
Copy it to your packages directory:
Mac OS X: ~/Library/Application\ Support/Sublime\ Text\ 2/Packages/
Linux: ~/.config/sublime-text-2/Packages
Windows: %APPDATA%\Sublime Text 2\Packages