The new CSSLint functionality works the same as JS Lint, through Tools->Clientside->CSS, or through the shortcut Alt+Shift+U. If there are any errors you will see them in the bottom panel under a tab called Clientside Issues.
Now the next step is to get Clientside plugin working with Gedit 3. I know this will take a bit of reworking, but honestly I'm still using classic gnome, not Unity or Gnome3. While I would like to have a version for both, its not priority at the moment. I plan to start using either Unity or Gnome 3 with Ubuntu's next release. I felt it was too soon for this round, and didn't have time for a cut in productivity trying to get things straightened out.