For some user enabling effects using "desktop-effects" fails with error. If so, run in terminal:

$ desktop-effects 

and try to enable them once more. If you receive something similar to:

compiz (core) - Fatal: No sync extension
Xlib: extension "SHAPE" missing on display ":0.0".
Xlib: extension "SHAPE" missing on display ":0.0".
Xlib: extension "SHAPE" missing on display ":0.0".

That means you should add  

Load "extmod"

to your /etc/X11/xorg.conf file in Module section and restart X server. That should solve a problem.

More info:
http://www.nvnews.net/vbulletin/showthread.php?t=102570
http://www.nvnews.net/vbulletin/showthread.php?t=102374