Change the gconf key with
gconftool-2 --type string --set /desktop/gnome/session/required_components/windowmanager compiz
You can go back to the default Gnome Metacity window manager with
gconftool-2 --type string --set /desktop/gnome/session/required_components/windowmanager gnome-wm
If this fails You can simply add compiz --replace to your startup applications. Name the entry what you want, give it whatever description you want, but make the command
compiz --replace
Fonte: link