You must catch the eight glowing rabbits found in the Character's Mini-Games in the following order:

Rabbit #	Caught With	Location Found
1	Luigi	In the underground hallway by the Lethal Lave Land course.
2	Yoshi	In the garden outside of the castle.
3	Wario	In the hall of the Tiny-Huge Island course.
4	Mario	On the staircase heading up towards the Tick-Tock Clock and Rainbow Ride course.
5	Luigi	In Bob-Omb Battlefield Hall
6	Wario	In the hall of the Tiny-Huge Island course (where you found it with Wario)
7	Mario	In the Jolly Roger Bay Hall.

Use the key to unlock the White Door to the right of Wario.