Your xbox should have an IP.
Within the XBMC settings, network youll find it.
Depending on if you are using a router or direct-connection, you will need to change the stuff found here.
Static/Dynamic etc etc. (it should match the info found within your OSX system network prefs, except the IP address of course)
From there, youll get the IP of the xbox.
Use any ftp program, (Transmit, fetch etc), to connect to the IP using xbox/xbox as the log/pass.
The best way to do this is to use a router. Within the router settings, from a browser window, you assign the MAC ID of the xbox to a permanent IP address. and use that IP to log in.
The settings within your router and the xbox network settings need to match, like the subnet mask etc etc.
xbox-scene.com is still the best repository of info.