you need to either
1. mod ur hdd and use SATA cables to transfer data direct from pc (links avaiable from this forum)
2. buy xploder kit and use memory card to transfer data (greatly detailed in this forum)
think these are the only 2 ways to do it. but even if there are more, they all involve buying new kit to access the data files stored on your xbox.
i hav been interested in doing this myself, used to do a lot of modding on FIFA2k *spit* when i had it on pc. it was so easy (and saitsfying when your kits, faces and music worked) but with consoles it's hardly worth it, especially the 360, as its so long winded and expensive. PS3 sounds easier to mod seeing as it allows you to use any usb device to save and transfer data (take a leaf MS)
Even when you get it working, the edit files are limited to only 8 kits total for the entire game, badges and sponsors included. you cant associate 'original' faces with new created players (if you ever wanted to dupliacte a player - which i do). none of it seems worth it to me. if i cant hav all the proper premier league kits then i dont want any.
i might go and get this on pc and have more fun modding it than playing the damn thing. PES has been a broken series of games for as long as i care to remember and i've been playing it since it was ISS on the SNES. still, its better than FIFA *spit* (minus the official licenses)