I just can\'t figure out what is wrong with my setup with Susy. I am on OS X 10.7.4
I have followed along here: http://susy.oddbird.net/guides/getting-started/
While it may not be "the correct answer" I had the same error, and realised I was trying to build with SASS, instead of using the Compass build system.
A bit silly I know, but hopefully if someone makes the same mistake, it could be helpful :D/