First of all I followed this description. My scss executable is located in "/home/user/.rvm/gems/ruby-1.8.7-p374/bin/scss". I use the rvm, because I need several ruby versions for my projects (in this case ruby 1.8.7).
A test in the IDE throws a error "/usr/bin/env: ruby_executable_hooks: NO such file or directory". In terminal it works as expected ($> scss test.scss test.css).
In the PhpStorm tutorial I found no solution. But this link point me the right way. You have to set up PATH and GEM_PATH in the file watcher dialog.
In my case:
PATH : /home/carlos/.rvm/gems/ruby-1.8.7-p374/bin
GEM_PATH : /home/carlos/.rvm/gems/ruby-1.8.7-p374
Please replace ( /home/carlos/) with your correct path.
I hope this will help somebody.