Instructions
Select "Grand Prix" from the "Mario Kart Wii" main menu.
Select "50cc."
Select a character, kart and control scheme.
Select the 50cc Star Cup.
Finish the 50cc Star Cup in first place. The Star Cup has four tracks. When you finish the first track, a message will inform you that King Boo is unlocked. He will appear in the bottom-left corner of the character-select screen.