No problem...go to your XFCE Settings Manager (xfce menu -> settings -> settings manager, in debian), then click keyboard, then the shortcuts tab. Make a new theme if you need to. You probably can't edit the default theme. Add a new command or shortcut. When it asks for the command to use, just type:
It will then ask for a keyboard shortcut I used Ctrl+p, as it seemed logical, then click okay. Restart XFCE (logging out is the easiest way), and you will be ready to go. You may only need to restart the panel, but I'm not sure.
You can type that command anywhere, (terminal, run dialog, etc.), and the places menu will come up.