At the main menu, press Up, Down, Right, Right, Left, Up, Up, Down
All tracks:
At the main menu, press Up, Up, Left, Down, Left, Right, Right, Up
Story mode:
At the main menu, press Down, Left, Up, Right, Right, Up, Down, Left
Arcade championships:
At the main menu, press Right, Left, Up, Right, Down, Down, Left, Left.
Multi-player championships:
At the main menu, press Left, Down, Left, Up, Right, Left, Down, Right