Instructions
Connect the "Pokemon Diamond" cartridge to your Action Replay and connect your Action Replay to your Nintendo DS.
Turn the Nintendo DS on and input the single following code:
94000130 FCFF0000
B21C4D28 00000000
B0000004 00000000
10001170 000001E4
D2000000 00000000
Press "Start" to start the game. Select your save file.
Walk around in the grass until you encounter a Palkia. Use a Pokeball to capture it. If you travel to the patches of grass near Twinleaf Town, the starting area for "Pokemon Diamond," Palkia will be at a lower level than if you travel to other areas.