To unlock Luigi's Mini-Games, catch rabbits at the following locations with Luigi:

Memory Master-Near the entrance to Wet Dry World.
Lucky Stars-Near the entrance to Bob-omb Battlefield.
Mario Slot-In the secret room with the Mario painting.
Mushroom Roulette-In the area with the columns used to drain the moat.
Pair-a-Gone And On-Outside in the hedge maze.
Picture Poker-In the courtyard above the entrance.
Super Mario Slot-On top of the Castle. (You need 150 stars)