Once you visit Stark Mountain, return to the 'Members Only' house to see Buck standing outside. It turns out that this is his house, so he invites you inside, where his grandfather welcomes you to the Battleground. Now you're free to come in whenever you please.
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You will find Darkrai at Newmoon Island but you need a membership card and go to that one house that is locked.
you use the radar in your pokech' and go to the front of the hotel near valor lakefront
its the top trainer cafe, u cant open it until after you finish the events at stark mountain. once u save the magma stone u can go back and visit the cafe and battle gym leaders each day
the ds isn't region locked so i would think that it would work, but i haven't tried yet.
If you are talking about the one in Canalave City, which is the only one I can think of, you have to go to a special event and get a key. Inside is where you can find darkrai.