you have to get it in an event. it is called the members pass, and once you get it and go in the house a man will put you in a bed. once you sleep you will wake up on newmoon island, where you can catch Darkrai.
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You can only get in the Inn with the Member's Card which is a key item that is an event item. Only Nintendo can give out the event items. They're not available anywhere in-game. unless you cheat with an ar
You can't sorry the only way to get it is with action replay
You can only get the secret key in Pokemon Platinum
platinum is coming to America in 2009 ask someone in japan
i think the key is "ENTER"