How to Unlock World 2 Cannon in New Super Mario Bros. Wii: There Isn't One—World 2 Unlocks Automatically

There is no cannon that unlocks World 2. World 2 unlocks automatically upon defeating the World 1 Castle boss (Larry Koopa).

Cannons in World 1

• World 1-5: Secret exit (after midway, enter green pipe above red Koopa) → Cannon minigame → Shoots you to a Star Coin (not a world unlock).
• World 1-Castle: Cannon at end launches you to boss door. Not a skip.

World Unlocks Overview

• World 1 → Beat 1-Castle → World 2
• World 2 → Beat 2-Castle → World 3
• World 3 → Beat 3-Castle → World 4
• World 4 → Beat 4-Castle → World 5
• World 5 → Beat 5-Castle → World 6
• World 6 → Beat 6-Castle → World 7
• World 7 → Beat 7-Castle → World 8
• World 8 → Beat 8-Castle (Bowser) → World 9 (Star World)

Tip: Secret exits (red flag) in some levels unlock shortcut paths on the map (dotted lines), but never skip entire worlds.