Instructions
Graduate from the Wigglytuff guild. To do this, you must complete a very large dungeon that takes place inside a building in the Mystifying Forest. There are 14 floors to the dungeon, with a set of Pokemon waiting to battle you on each floor. The difficulty of the battles increases as you advance through the dungeon, and you will end up battling nearly every type of Pokemon.
Restart the game to its main menu.
Press the "A" button on the "Wonder Mail" option. A window will appear asking you for a code.
Enter "X 7 = C J 8 + 3 F R W K + 9 1 9 H % 0 9 Y + F +" into the window, and press "OK" to submit the code.
Load your saved game. When prompted to choose your next mission, Midnight Forest will appear in the list.