Unlock Multiplayer mode by beating Tiny Wario's stage in Single Player, then go to the Main Menu and select Multi.

Effect - Code

Balloon - Play a game of Lifeline
Bomb - Play a game of Survival
Bungee Buddies - Play a game of Star Nose
Darts - Unlock Multiplayer (Beat Tiny Wario's Stage)
Lifeline - Play a game of Bomb
Star Nose - Play a game of Darts
Survival Unlock - Multiplayer (Beat Tiny Wario's Stage)