Things You'll Need
Instructions
Power on your Nintendo DS with the Action Replay device inserted. Select "Pokemon Diamond" from the game list.
Select "New Code." Name the code anything you like, such as "Azure Flute" or "Arceus."
Enter "94000130 FCFF0000," "B21C4D28 00000000" and "B0000004 00000000" on the first three lines. On the fourth, fifth and sixth lines, enter "0000AA3C EDB88320," "2000AA68 0000000A" and "D2000000 00000000."
Start the game. You now have the Azure Flute in your inventory. Head to Hearthome City and walk west to Mt. Coronet. Enter the cavern.
Climb to the top of Mt. Coronet. When you arrive, you are asked if you would like to play the Azure Flute. Answer "Yes." A staircase to the Hall of Origins appears.
Walk up the staircase. Prepare your Pokemon team for battle. Fight Arceus like you would any other Pokemon you wish to catch. When its health bar is red, throw the best Poke Balls you have at it. If you have a Master Ball, throw it to capture Arceus in one attempt.