I had the exact same problem. Got some hints from this thread and got me headed in the right direction. Not sure what OS you are using, but I am using Apple TV 2 recently jailbroken with EDEN. Anyway the filename that is mentioned throughout this thread is wrong, and so is the location. Here is where I found the file I needed to edit.
Inside of that folder you will find a file called “mediasources.xml” that holds the config that you are referring to. The file name mentioned earlier, “sources.xml” holds the config for what you would see under the Video heading in XBMC.
I used ssh from a linux terminal to get access to the file. sftp://firstname.lastname@example.org/private/var/mobile/Library/Preferences/XBMC/userdata. This can be done from “terminal” or “Nautilus”. Nautilus is easier cause you get the GUI you were looking for. You can also do it from Windows. Download a program called WinSCP. It is very straight forward. Just enter the ip address of your apple tv and the user and password. Mine is freshly jailbroke, and the user is “root” password is “alpine”.
Hope this helps you, and if not someone else. It was driving me crazy too.