You can get the member card in diamond and pearl but the event for it in platinum is august 3rd to September 13th (you have to have wi-fi connection to do this)
at a Nintendo event
Mystery Gift or using Action Replay
You can get it through the Nintendo wifi.
you have to get the members card from mystery gift.
you have to obtain the members card through mystery gift.(it may be possible trough AR.)
Not without cheating
With the Japanese event or mystery gift
There is not code to get one without amember's card. Well there is. Go to mystery gift and search
the event is over but you used to be able to get the members card which allows you to catch darkrai you can still get the members card with action replay
It comes with the Mystery gift...
By Mystery Gift,by Wi-Fi.Hurry,it stops near September 18.
you get it through Nintendo event or mystery gift on Pokemon pearl platinum or diamond