Instructions
Start the game and choose Piplup, Turtwig or Chimchar as your starter Pokemon. Win the Coal badge from Roark of Oreburgh City. You need this first badge to activate Wi-Fi trading in "Pokemon Diamond," "Pokemon Pearl" or "Pokemon Platinum."
Take the descending escalator at the PokeCenter in Jublife City. A lady will give you the "Little Red Code Book" after speaking to you. Save your game and soft reset the Pokemon game. Select the third option on the main menu instead of "Continue."
Select "Search for Access Point" to set up your Wi-Fi Connection. Return to the main menu and choose "Continue." Head west of the PokeCenter in Jublife. Enter the GTS building. Speak to the lady behind the counter and she will tell you how the GTS works. Choose the first option and you will teleport to the GTS room.
Choose to "Search" for a Pokemon. Select "Darkrai" as the Pokemon and choose what the level range of the Darkrai should be. Since Darkrai is genderless, you cannot pick what gender you prefer the Darkrai to be.
Check every Darkrai available on the GTS. Pokemon trainers have offered their Darkrai in hopes of receiving the Pokemon they want in return. If you have a Pokemon a person wants in exchange for the Darkrai, choose to trade it. For example, if a Pokemon Trainer wants a Ditto for his Darkrai, select the Ditto to trade from your Pokemon Storage System. After the trade, the Darkrai will be in your party or Pokemon Storage System.