Okay... generally i would recommend using FTP to add the files of XMBC to your Xbox and either running it as an application off your dashboard(avalaunch, evolx, unleash) or as a dashboard itself..
Read the online guide it will walk you through anything, please be patient and actually read and look through the guide for what you are needing to do, the whole burning a CD to run XMBC will also most likely be described on the XMBC guide...
good luck
important links:
http://www.xboxmediacenter.com/ --HOMEPAGE
http://manual.xboxmediacenter.de/wakka.php?wakka=Index -XMBC GUIDE!!!