To use cheats in Colin McRae 04 you have to get cheats that work specifically for the access code you have. To view your access code, look at the secrets section of the Options menu.
While we have cheats for a good number of access codes, we don't have them all. However, you can change your access code to one that does match our codes. To change your access code, delete the Colin McRae 04 auto-save file from your Xbox and restart the game. When the game asks you to enable auto-save say "Yes" and it will create a new save file with a randomly generated access code. You can repeat this as many times as you want to get an access code that matches ours.
Since this requires deleting your save file, you may want to make a back-up of the file on your memory card. Or, if you don't have any important information saved, you can just delete the existing file to get a new access code.