In pokemon platinum there is an event via wi-fi where you get the member card. Go to the sailors house in canalave city there will be a boy suffering a nightmare go talk to both of them and then talk to the sailor. He will take you to an island where you find Cresselia, after it vanishes it will leave a lunar wing or something. Go back to canalave city talk to the boy and wake him. Then go to harbour inn (the house on the top right) there will be a creepy man that will put you to sleep. You will wake up in an other island where you find Darkrai.
You need an action replay and get a wonder card. Then go to canalva city and go to the harbour inn and go up to the door. It won`t be locked and go into the bed and you will go to sleep and when you are dreaming you will find darkrai and when you catch it you will have it when you wake up.
Get the member card with th wifi event august 3 through september 13 2009
Get member card
Get the darkrai mystery gift then go to a poke mart talk to a man in a green suit he will give you a members card and the door will be unlocked and you will be able to get darkrai