Instructions
Clear the normal storyline in "Pokemon Diamond" or "Pokemon Pearl." This includes defeating both Team Galactic and the Elite Four, as well as encountering one of the three Legendary Pokemon.
Train your team to at least level 80 to prepare for the eventual battle with Arceus.
Add new cheats from the No$gba cheats dialog. Select "Cheats" from the "Utility Menu," and press the "Add New" button to add a new cheat.
Enter the code "94000130 FCFF0000 B21C4D28 00000000 B0000004 00000000 0000AA3C EDB88320 2000AA68 0000000A D2000000 00000000" to activate the Arceus event.
Press "L" and "R" simultaneously to make a new character appear in any Pokemart. The character will be wearing a green hat and suit.
Talk to the man in green to receive the Azure Flute, the event item normally only attainable through special Nintendo events.
Travel to Mt. Coronet on Route 207, which you may remember traversing much earlier in the game.
Climb Mt. Coronet until you reach the Spear Pillar on the summit.
Use the Azure Flute in your inventory to create a new staircase. Walk up the staircase to reach the Hall of Origin.
Approach Arceus to begin the battle. When Arceus is low, capture him with a Master Ball.