I have Sublime Text 2 version 2.0.2 build 2223 and found that toggle comment did not work either from the menu selection or the keyboard shortcut for command-/.
So I copied the C++ comments tmPreferences file:
~/Library/Application\ Support/Sublime\ Text\ 2/Packages/C++/Comments\ \(C++\).tmPreferences
~/Library/Application\ Support/Sublime\ Text\ 2/Packages/C\#/Comments\ \(C\#\).tmPreferences
And then edited the scope <key> to replace source.c, source.c++, source.objc, source.objc++ with source.cs.
I'm not sure if this is the preferred or recommended approach as I just started using Sublime Text 2 yesterday.