I have XBMC installed...Do you have a dashboard installed ? If your gonna ask whats a dashboard then this will help :
http://xbox-scene.com/
anything remotely related to xbox that site will have. The order is as follows:
1) Install a dashboard if there is none present. Easiest to install in Evolution-X
2) Setup Ftp on your xbox
3) FTP the contents of XMBC unzipped onto the XBOX.
4) Modify the xml file to see XMBC being reflected.
All of the above steps is there in the link I provided. Hope this helps.