Instructions
Find someone who owns a legitimate Darkrai and negotiate a trade. You can check various Pokemon forums for someone offering a Darkrai, or find a friend who attended a Darkrai Pokemon event.
Connect your Nintendo DS to your friend's system, or access the Internet using the Nintendo DS Wi-Fi functionality.
Enter the second floor of any Pokemon Center in "Pokemon SoulSilver."
Access the trade system and connect to your trade partner using his or her friend code. Each "Pokemon" player is assigned a permanent friend code after connecting to the Internet with any Nintendo DS game.
Complete the trade to receive your cheat-free Darkrai.