You need to take a special dream bridge from the special home that is ONLY accesible to your new legend, and then encounter victini on the special island. Victini needs help opening a treasure chest, and so you have to help it in a new game involving YOUR Zekrom! Do not worry. If you fail, it will return you to the special home but you CAN return right away to try again. Once you have beaten the game, Victini gives you the skin and you can exit the special home. Afterwards, you can still cross your usual dream bridge and go to the island/tree of dreams. Good luck, and enjoy!