Enter the following codes at the main menu where it says "Start Game":
Master Code: Hold L + R and press A, Y, Right, B, X, Down, Y, X, Left, B, A, Up
Max Stats: Hold the L + R then press Y, Y, Right, Y, Y, Down, A, A, Left, A, A, Up
Sticky Boards: Hold the L + R then press X, X, Right, Y, Y, Down, B, B, Left, A, A, Up
Mix-Master Mike: Hold L + R and press A, A, Right, A, A, Down, A, A, Left, A, A, Up
Mallora Board: Hold L + R and press A, A, Right, B, B, Down, Y, Y, Left, X, X, Up
Annette Board: Hold L + R and press X, A, Right, X, A, Down, X, A, Left, X, A, Up
Hints
Unlock Hidden Course: Win a medal in all Race levels.
Unlock Pipedream Course: Win a medal on all of the Showoff courses
Unlock Riders by obtaining the required amount of medals shown below:
Brodi: Win one gold medal in world circuit mode.
Zoe: Win two gold medals in world circuit mode.
JP: Win three gold medals in world circuit mode.
Kaori: Win four gold medals in world circuit mode.
Marisol: Win five gold medals in world circuit mode.
Psymon: Win six gold medals in world circuit mode.
Seeiah: Win seven gold medals in world circuit mode.
Luther: Win eight gold medals in world circuit mode.