I faced the issue that compass was not working on windows 7 64 bits and I solved by installing ruby and then the compass gem. Depending on your system, [you can download ruby here] (http://rubyinstaller.org/downloads/) Then don’t forget to set up the environment variables on windows I think you can do that directly from the installer, but also from Computer -> Control Panel -> Edit Environment Variables. Then on System Variables, add to the Path variable the ruby path,
Finally open a CMD terminal and write:
gem install compass
Now grunt should be working well:
grunt serve
Running "compass:server" (compass) task
directory .tmp/styles
write .tmp/styles/main.css (0.096s)
write .tmp/styles/main.css.map
Done, without errors.