Things You'll Need
Instructions
Using the Pal Park
Insert into your DS both your Pokemon Diamond DS cartridge and a GBA cartridge for Pokemon Fire Red or Leaf Green, or Pokemon Ruby, Emerald or Sapphire. The GBA game must contain Celebi as a captured Pokemon.
Start the game. At the title screen, scroll down and you should have the option to transfer Pokemon from the GBA game. Transfer Celebi and five others (Pokemon can only transferred in groups of six).
Continue to the game itself and head to the Pal Park. The Pal Park is on route 221. Use a Pokemon with the HM surf to travel southeast from Sangdem town to reach it
Enter the Pal Park. You will be given six Park Balls to catch the Pokemon within, which have a 100 percent catch rate. Walk around the Pal Park until you have located and caught Celebi and the other five Pokemon you transferred. Celebi can be found in areas of forest, but appears infrequently as it is a rare Pokemon and appears as it would in regular gameplay.
Transfer the Pokemon to your PC and save your game. You can now transfer Celebi from your PC to your Pokedex and use it as normal.
Using the Action Replay
Place your Pokemon Diamond DS cartridge in the top of the Action Replay cartridge and insert the Action Replay cartridge into your DS.
At the home screen, tap the "Code Select" button to get to the code screen.
Select "Add New Code" at the top of the code list. Enter a name for the code when prompted and select "Next" to get to the code entry screen.
Use the keyboard to enter the following code -- do not enter the slashes, instead remove them and start a new line wherever one appears:
94000130 fffb0000/ d5000000 000000fb/ b21c4d28 00000000/ b0000004 00000000/ c0000000 0000000b/ d7000000 00024620/ dc000000 00000006/ d2000000 00000000
Press the L and R buttons on your DS to activate the code and start the game. If entered correctly, Celebi should have replaced the first Pokemon in your party.