Are you running MATE or Cinnamon? If Cinnamon, it doesn't support screensavers (at least not yet).
Main: Intel Core i7 920 D0 @ 4.0GHz | Asus P6X58D Premium | 6 x 2GB Mushkin Redline PC3-12800 7-8-7-24 | EVGA GeForce GTX 560 Ti | 6 x 1TB WD Caviar Black | Mint 13 Cinnamon / OS X 10.7.3 Portable: Dell Mini 9 | OS X 10.6.7
I use maya cinnamon, what i do is delete the default gnome screensaver with software manager and then install xscreensaver and the gl extras. To change the screensaver just click screensaver from menu. Glmatrix look best for me