Things You'll Need
Instructions
Insert the copy of Pokemon Diamond or Pearl into one Nintendo DS and turn the system on.
Select the "Mystery Gift" option from the menu and press "Receive Gift." If Nintendo is currently sending out "Membership Pass," your DS will automatically download the Membership Pass to your copy of Pokemon Diamond or Pearl. You will need a membership pass to access the house where you catch Darkrai.
Start the game and make your way to Canalave City. Go to the motel and hand the "Membership Pass" to the attendant. He will let you stay at the hotel for the night. A short scene will occur during which you will be visited by Darkrai. The location of Darkrai will now be marked on your map.
Travel to the southern tip of Canalave City and talk to the sailor at the docks. He will take you to New Moon Island where you can battle and catch Darkrai. Darkrai will be at level 40. Travel to a Pokemon Center and save your game. Talk to a trade attendant.
Place the copy of Soul Silver into the other Nintendo DS and turn it on. Make your way to any Pokemon Center and talk to the trade attendant.
Allow a chance for both Nintendo DS systems to link by selecting "Trade" from the menu after talking to the trade attendant.
Enter the trade room on both games and select the "Trade" option again. Select a Pokemon to trade to your copy of Pokemon Diamond or Pearl. Select Darkrai to trade to your copy of Soul Silver. Press "Confirm Trade." Once the trade has been completed, you can save and turn off both systems.