Do you have an mod chip and are you able to ftp to your xbox?
If your answers on both are yes do this:
1. Transfer the folder XBMC to: E:\APPS\XBMC\ on your xbox.
2. If you want to have XBMC to be your default dashboard copy the default.xbe file from the XBMC folde to your C: drive and replace the original file (back it up before you overwrite it!)
If you are trying to put XBMC on a softmodded box read this:http://forums.xbox-scene.com/index.php?showtopic=406656
Hope this helps...