Action Replay is a game-code device that allows you to input cheat codes and change the way you play a game. One of the uses for an Action Replay is to catch any Pokémon, like Mew, in "Pokémon: Pearl." It's a simple process that involves entering the code and then simply playing the game to receive a Mew from the fossil man in Oreburgh city.
Place the Action Replay into the Nintendo DS, and place your copy of "Pokémon: Pearl" into the Action Replay.
Turn the Nintendo DS on, and insert the following code when prompted:
94000130 FCFF0000
B21C4D28 00000000
B0000004 00000000
10001170 00000097
D2000000 00000000
Press "Enter" to start the game.
Make your way to Oreburgh, and talk to the fossil man in the upper-right corner of the city. Ensure you have an empty slot in your party when you talk to him. If done correctly, the fossil man will give you a Mew.