What “ozjd” wrote. XFCE re-write its configuration files when it quits, ignoring all changes that happened behind its back. So you need to do them while XFCE is not running.
(I don’t know if this is true for the just released XFCE 4.8, but it’s irrelevant to this discussion.)
Edit: Just noticed the original post’s date. Sorry for being late/replying to an old post (hopefully, the original poster already fixed his/her problem).