There are a couple of ways that you can go about this.
1, is by using a cheating device to get the Membership Pass that activates the Darkrai event.
the 2nd, is going to Cresselia's island and surfing to the east until you come to Darkrai's island.
the 3rd, is trading with someone that has it.
EDIT:
I forgot the 4th and newest way of getting Darkrai. With the newest Pok�mon Ranger game is a mission for Darkrai, once completed this Darkrai can be transferred to Diamond or Pearl.
« Last edited by Mummification on Jan 7th 2009 »