Comment 8 for bug 366828

I'm not using xfce for a while now but as far as I remember there are two places in xfce configuration where you can define keyboard shortcuts:

in 'keyboard settings' that can be accessed either from menu->settings->keyboard settings or menu->settings->all settings
in window manager settings

Try to add shortcut that is mentioned in one of previous posts: Alt+KP_2 and just run some application as an action.

Sorry but I don't remember that well what the exact problem was.