I finally know how to get there!

It depends on your pokemon party what the guy say. Every pokemon has a number(not a pokedex number but just a number). Each time you deposit a pokemon from your party or do a box activity, the number chance. So your party has 6 numbers (if you have 6 pkmn in it, but you have to or else this cheat wont work). If the 6 numbers ar correct then you can go to mirrage island. I know that female pokemon have good numbers and that Latios + Latias have good numbers. Mirage Island isn't big. You can only find a gold berry and a Wynaut + some other pokemon there.