It was an event in 2009 that you can't access again, sorry.
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you play the game for a long time
In Diamond/Pearl/Platinum you have to get the national dex before you can enter the pal park.
If you have a game shark for your system check the cheats.
You have to get a Mystery Gift or a cheat device like, Action Replay or Gameshark. To get in there you have to have a Key Item called a Membership Pass. Once you get in there you can catch a Pokemon named Darkrai.
You cant open the door.unless you have the Members pass which allows you to get Darkrai. and you can gat the pass only by a wifi event. hope this helps :)