You must start a new save file and proceed into the game until you reach professor Oak. The professor will give you your first pokemon to train, and it will have a name other than mew (bulbasaur, charmander, squirtle). Once you receive the pokemon, it will ask you to nickname the character. Name the pokemon mew and it will be listed as the 151st pokemon in the pokedex as "mew 151st." The pokemon will show up and fight as mew from this point on.