First,you have to complete the Unova Pokedex.Then you go to Prof. Junipers lab in Nuvema town and she''ll give you the pass to go to the Nature Sanctuary.Now all you have to do is go to Mistralton city and enter the Blue Building.
Source
First you must see all the Pokemon in the Unova Pokedex. Next go to see Prof' Juniper. She will give you the Permit. Next go to Mistralton city. Talk to the lady behind the counter inside the Building catch planes. The next bit is a surprise!!!
