so first you go to dlc shop it will say update and show mewtwo keep on scrolling until it says continue then there will be an mewtwo for wii u and 3ds choos that . i will update wen i get more info.
to find mewtwo you need to go to the garden where you find the 8th gym leader if you keep looking you may find a cave if you do try to find a way to get into the cave in there will be mewtwo waiting to battle go up and press a to battle. i hope you cach mewtwo!