Instructions
Insert the Action Replay cartridge into your Nintendo DS device and attach the "Pokemon Pearl" game to the Action Replay.
Click the "Add a New Cheat" option and turn on the "Walk/Run Anywhere" cheat by highlighting the code and pressing the "A" button on your DS device. If you do not have the code, enter the following code:
b21c5b48 00000000
da000000 0000210c
d3000000 00000000
d7000000 023ffe18
d2000000 00000000
94000130 feff0000
923ffe18 ffef0010
b21c5b48 00000000
dc000000 0000210c
d9000000 00000000
d4000000 fffffff0
d6000000 00000000
d2000000 00000000
Press the "Start" button to start the game. Walk toward Victory Road and go through the cavern until you reach the top floor.
Speak with the girl, who will ask you for your assistance. Allow her to follow while you head through the south walls (press the down arrow) until you reach the Mystery Zone.
Ditch the girl by walking through the wild grass until you reach a wild Pokemon. Battle the wild Pokemon and defeat it. Once you have exited the battle screen, the girl should disappear. You must get rid of the girl before catching the Missingno creature because your cheat will not work with her following you.
Continue south and exit Victory Road. You will see a Pokemon Center located to the left of the road. Since you have the "Walk/Run Anywhere" cheat on, walk on the water left of the Pokemon center until you encounter a Missingno.