Enter SQUIRRELDOG in the character completion field.
All Machines
Go to the cheat menu at options and enter at all machines 'MINIGAMES'.
Easy way to beat stunt challenges
When you get to the stunt area press circle to enter the event, then go to a ramp and keep on doing backflips. (By pressing at the lip of the jump up than down) Then keep doing it repeatly until in purple it says Opjective Complete.
Get all bonuses
Go to the cheats menu and enter: CLAPPEDOUT.
Open Up Pro Physics
Enter SWAPPIN in the character completion field.
Unlock 500cc bikes
First go to the cheatcodes section and put in the code "BIGDOGS" (without the quotation marks) and you will get big bikes.
Unlock Freestyle Tracks
Enter L1, L1, X, X, X, X, Square, R2, R2, L1.
Unlock supercross tracks
To unlock all supercross levels go to the cheat codes menu and put in the word "STUPERCROSS" (dont put the quotation marks in though.)