After you solved all 50 A puzzles, the game gave you the code to visit "Another World" and the evil sorcerer unlocked.
In Puzzle Mode, Select 1-A, after 2-A, all the way to 5-A. After finish the last puzzle in 5-A, you'll get the cheatcode and the end credits. Then restart you DS and you'll be able to play as the Evil Sorcerer.
Sound test
At the main menu, press Select, A, B, Left, Right, A, Select, Right.
Unlock Dark World
When you finished all the A worlds in the light world with DEV enter the following cheat code at that title screen: A, Left, Right, A.