If you mean the card to get rotom form then that was event.
If its for darkrai then go to pokemonplatinum.com ( if you have English version) and then go to Pokemon then darkrai. it will tell there.
hurry though the event ends September 13
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Have your friends Pokemon hold the member card and trade it.
The Member Card is an event exclusive item on Pokemon Pearl. However the event to give the Card out is no longer being held. This means the only method of getting the Member Card is with a cheat device.
You have to get the member card event.
The member pass was only distributed via a Nintendo event. The Member pass as i was know before release was officially called the Member's Card. This card was used to obtain Darkrai
I believe there was an event for darkrai/member card at Toys 'R' Us. Unfortunately, the event is long gone.