Go to the Profile Editor option from the main menu, choose Unlock Items, and then Cheat Codes.  Type in one of the following codes to unlock the corresponding cheat:

Cheat	Code
Master Code	IGIVEUP
1,000 Profile Points	GIMMEPTS (then press X)
All ATVs	SHOWROOM
All Championship Events	GOLDCUPS
All Games	GAMEON
All Tracks	TRLBLAZR
All Equipment	THREADS
Disable Wrecks	FLYPAPER
Switch Aspect Ratio	WIDESCRN
Tougher A.I.	BRINGIT
Wheelie Points	GETONUP