To enable the codes to work, you must do the following:
Race the full circuit on intermediate. You must place first. This will allow you to access 4 new tracks. Race the full circuit on professional. You must place first again and you will be able to access 2 more new tracks. Place first again on the full circuit on professional. This will allow access to 1 final track. Place first on the full circuit on professional. Go to the title screen. It should say "CODES ENABLED".
Type in the codes:
ZOWIEZOOM = Unlimited turbos
2XSTUNTS = Double Stunt points
CONTORTIONIST = Agility
JETPACKSPECIAL = Rocket
ZIPPYNODRAG = No resistence
SWOOSHSKATE = Ice Racing
SCREECHNOW = Air Brakes
YAHOOLOOKIE = Show off camera
BRAINIACPLUS = Inverted Camera