All I can tell you is that the inn is only for the Darkrai event. You need to get the mystery gift "Member Card" via Wi-Fi. Go to the Jubilife tv station's 3rd floor. You should see a man with glasses next to a blonde women. Talk to him for an interview. Say "Everyone, Happy" and then "Wi-Fi, Connection". Save your game, reset, and at the main menu you will see "Mystery Gift". Choose it then pick "receive Via Nintendo Wi-Fi". Go TRY to open the harbor inn. I don't know why it won't open for me, but apparently this is all you have to do to catch Darkrai.
