ok mate, you need halo 2 running from your XB's hdd, (E:\games\halo 2\ ) is where mine is stored, then you can acces the map files, and mod, for stuff like dashboards and configuring your XB so you can load the game from hdd go to
www.xbox-scene.com
once your game is on the hdd and you can load it from there, you can transfer your maps with FTP to your pc,
for .ppf files get an app called PPF-O-Matic
and get an app like CH2R for making your own mods and Resigning.
if you need any help with things i have just mentioned, hunt around, all the info is on here somewhere.