First of all use the router if it has a DHCP server on it (must of them have). Make sure you set the XBMC network settings
- not static
- use dhcp
something like that. Don't them by heart. If you start your xbox the ip of the xbox should be shown in the first screen of XBMC.
Question: does your xbox boot up in XBMC?
BTW you posted in the wrong forum! This is XboxMediaPlayer
