Things You'll Need
Instructions
Insert the Action Replay cartridge into the Nintendo DS. Make sure that the power is off before you insert the cartridge.
Turn on the power to the Nintendo DS. Wait for Action Replay to boot up.
Remove the Action Replay cartridge when prompted on the top screen of the DS. Leave the Nintendo DS on during this process.
Insert the Pokemon SoulSilver cartridge. The top screen of the DS will still read "Action Replay," but the title below should read Pokemon SoulSilver. The bottom screen will feature a bright red button reading "GO," and a green check mark.
Select the green check mark. A list of codes available for Pokemon SoulSilver will appear on the top screen. Scroll through the selection of codes and check off "Mew" from the Pokemon list.
Press the "GO" button on the bottom screen. This will end Action Replay and start Pokemon SoulSilver. Mew will be available in your Pokemon PC box in game.