When you try to choose a preset Visual Effect in the Appearance menu, what happens? Any errors?
Keep in mind there is a bug where Metacity fights Compiz for control. The fix is to uncheck compositing_manager in (Alt-F2)->gconf-editor->apps->metacity->general.
If its not the bug, then please give us more of what happens. Maybe some output of /var/log/syslog or /var/log messages when you try to enable it.