Go to the options and select cheats. Put in the code and your should here a confirmation sound. Then press "B" to exit the cheat menu.
All cheats are case sensitive.
Level select: GimmeGimme
Perfect jumps: StickiT
Super spins: WhirlGig
Low gravity: MegaLeg
Extra player and board: RidinwRaven
Another extra player: ChillingwSteezy
Dont hit trees: buzzsaW
Go really fast even uphill: ZiPster
Amped
