To unlock extra normal levels, do this:

Effect - Code

Mario 5, 6, 7 - Complete Mario's 1st 4 Levels.
Mario 8, 9, 10 - Complete Mario's 5th, 6th, and 7th Levels.
Wario 5, 6, 7 - Complete Wario's 1st 4 Levels.
Wario 8, 9, 10 - Complete Wario's 5th, 6th, and 7th Levels.
Wario's Super Picross (Wario 1, 2, 3, 4) - Complete Mario's 1st Level.