first go to extras and go to the thing that sez cheat and type one of these in

or name your self one of these what ever works for you

ELPUEBLO - Unlocks the El Diablo
GIEVCARPLZ - Unlocks the School Bus
GIEVEPIX - Unlocks all cars (and 1,000,000 credits)
GIVEALL - Unlocks all tracks
GIVECASH - 1,000,000 credits
RAIDERS - Unlocks the Big Rig
KALJAKOPPA - Unlocks the Rocket
RUTTO - Unlocks the Pimpster
WOTKINS - Unlocks the Flat Mobile