You must have a dex entry for all the Unova (BW2)(Excluding event Pokemons) dex and get a Liscense to see him. Serebii has provided a nice description:
After you have seen all the Pokémon in the Unova Dex, you will receive a License. This item, when shown to the airport attendant in Mistralton City, takes you to the Nature Preserve, a place filled with wild Pokémon. However, in the middle of this sanctuary is the rare Shiny Haxorus.