Here are the ways to get Darkrai in those games:
* Pokémon Platinum: You can get Darkrai by visiting the Newmoon Island after completing the main story.
* Pokémon SoulSilver: You can get Darkrai by connecting with a Pokémon Platinum game that has Darkrai.
* Pokémon Black/White 2: You can get Darkrai by connecting with a Pokémon Platinum or SoulSilver game that has Darkrai.
If you want to get Darkrai in HeartGold, you'll need to transfer it from another game. You can do this through the Nintendo Wi-Fi Connection, but this service is no longer available.
You could also try trading with someone who has a Darkrai in another game, but this would require finding someone willing to trade.