To access the cheat menu, select career mode with the X button.

1. Press right on the D-pad until "options" is highlighted.
2. Press down once then press right until "cheat codes" is highlighted.
3. Press X to select the option to enter cheatcodes for the game.
4. The cheats are case sensitive.  Once the cheat is input press the accept button onscreen. 
5. The cheat will disappear from the box.  Press the triangle button to get back to the main menu.
6. Start a race and the cheat will be enabled.