1. Find where your gems live by running sudo gem environment in the command line, Then look for the gem paths.
2. Go into Codekit prefs, click on Sass/Scss
3. Click on “Use the Sass Executable at this path:” and then navigate to the bin folder at the gem path you found earlier.
4. Select sass.bin
5. Move over to the Compass tab and follow the same steps, choosing your local copy from the bin folder in you gem path.
6. Refresh Codekit and voila!

– Source

Step 5 is missing on the Foundation Documentation Page, which was the reason, why my CodeKit App and Foundation did not work together.

Without choosing my local copy of Compass gem, I always got the following error message:

Compass was unable to compile one or more files in the project: LoadError on line 2 of /Library/Ruby/Gems/1.8/gems/zurb-foundation-3.0.9/lib/ZURB-foundation.rb: no such file to load — foundation/version
Run with —trace to see the full backtrace

Now, everything works! :-)