To activate a cheat code, go to the opening menu, click "Create Player", and type the cheat code in the name box. Click "Done", which will play a sound, then enjoy the cheat code's effects in the game!

Gulliver - Turns the cars into minature remote controlled racers, complete with antenna!
Smash Up - Demolition Derby
Fuzzyfuz - Enables cops in Quick Race mode, though they aren't on the Monte Carlo circuits.
Dakar - Rally style physics
yraGyraG - Gives all cars 993 physics
fetherw8 - Makes all the cars heavier