Type  them at the frontend of the Game or pause the race and type
in the cheats.

MACHTHREE  a new car with max speed of 600 km/h (in every race u select).
IMPACT     unlimited ammo in deathmatch.
OVERKILL   weaponmode in championship.
ALLOFF     cheats off.
 
     Enter in the game or in pausemode BUGGYBOY ...

Now you are in Debugmode following codes are in:

A - Autopilot youre car drives allone (besttimes)

F - Displays Framerates

G - Switch between the cars (be sure that you have pressed A before)

I - Informations system ( how wave is active ...and so on)

P - Sreenshoot