Go to Options, select Game Status and press A. hold L+R on any 
character and you will have a space to put codes in.
Note: you enter the letters by pressing Up or Down with the 
D-Pad and move to the next letter by pressing right on the Pad.
Note2: You must hold L+R before EVERY CODE to enable the space.

ELBICNIVNI ----  Invincibility
RAPID_FIRE ---- Rapid Fire
NO_GRAVITY ---- Makes car go higher.
GO_MONSTER ---- Big Wheels
LLA_KCOLNU ---- All Cars
LLA_DORTOH ---- Max Added Stats
BLAST_FIRE ---- Makes Missile Stronger
JACK_IT_UP ---- High Suspensions