To enable codes you must:

     Place first on the full circuit on Intermediate. This will open 4
new tracks.
     Place first on the full circuit on Professional. This will enable
2 more tracks.
     Place  first on the full circuit again on Professional. This will
enable the final track.
     Place first on the full circuit again on Professional. You should
now see the words 'Codes Enabled' on the title screen. Now you can use
the codes - just type them in at the title screen...

CONTORTIONIST           Super Agility
JETPACKSPECIAL          Rocket Racer
2XSTUNTS                Double Stunt Points
ZIPPYNODRAG             Zero Resistance
SCREECHNOW              Air Brakes
YAHOOLOOKIE             Show Off Camera
ZOWIEZOOM               Unlimited Turbos
SWOOSHSKATE             Ice Racing
BRAINIACPLUS            Inverted Camera