To activate a cheat, just edit one of your driver's names to the following:

NMRCHTS disables all cheats
NWHLS no wheels
NCHSSS no chassis
NDRVR no driver
PGLLRD play with all red powerups
PGLLGRN play with all green powerups
PGLLYLLW play with all yellow powerups
MXPMX all powerups maxed out
FLYSKYHGH permanent rocket boosters
NSLWJ don't slow down on rough terrain (grass, sand, etc)
LNFRRRM Mirrored Rocket Racer track
RPCRNLY Grapple powerups only

Cheats are disabled in circuit mode. Also, the cheats affect all of your opponents, not just you.

Happy racing! :P