Getting Mewtwo in PokéPark Wii: Not Possible—Not in the Game

You cannot get Mewtwo in PokéPark Wii.

Mewtwo is not a playable character in PokéPark Wii. The game focuses on a large roster of Pokémon, but Mewtwo is not included in the lineup.

You might be thinking of a different Pokémon game, as Mewtwo is a popular and powerful Pokémon featured in many other titles across the franchise.