If you have a transfer pack and red, blue,yellow version this is the cheat you need.
First, catch a mewtwo in red, blue, or yellow version with your master ball. then save and quit.
Then it will bring you to that menu where stadium and all that other stuff thats there and go to prof.oak's lab and switch Mewtwo to N64's Pak.then when you start a new game mewtwo will be there.
If you want to use Mewtwo in POKEMON STADIUM switch it to your pack.
(be sure you put mewtwo back)
Added by: Yu-Gi-Oh , ID#1893