Things You'll Need
Instructions
Insert the Action Replay into Slot One of your Nintendo DS. Insert the "Pokemon Diamond" game.
Power on the DS. Select "Pokemon Diamond" from the game list. Choose the "New Code" option.
Give the new code a name that helps you recognize it, such as "Steal Pokemon" or "Capture Trainer's Pokemon."
Enter "9223B5FA 00002101" on the first line, "1223B5FA 00002100" on the second and "D2000000 00000000" on the third. Save the code.
Load the "Pokemon Diamond" game. Speak to any trainer you have not battled before to initiate a battle with them.
Wait for the trainer to use the Pokemon you want to steal. Attack it until its health is low and then throw a Poke Ball, Great Ball or Ultra Ball at it, just like you would to catch a Pokemon in the wild.