You would have to wait for a new mystery gift to get Darkrai, it's the only way without cheats.
Darkrai.
Get the key from Mystery Gift and open the door in caveport that is locked.
Nope.
im pretty sure you can't catch it without cheats, its liek shaymin or acreus
i know how to get nearly every Pokemon without a cheating device. do you know how to get Deoxis on emerald Or is there a way to get Darkrai on diamond without cheats?
you cant i think you can not
i think you can only get it at a Pokemon event or with Pokemon ranger 2 (with out cheats.)
Darkrai can be found at Newmoon Island, but you can't do it without cheats. If you use the walk through everything cheat and you are at Full Moon Island, where you can find Cresselia, you have to walk to the right until you get at Newmoon Island. There you can interact with Darkrai.
http://www.youtube.com/watch?v=YRKPj4tcybE
You can't get him without events or cheats. For Darkrai, Manaphy and Arceus you need a event to capture him.
You needed a Nintendo Event to get an item to trigger an event to catch him, but this is unlikely to return.
You could trade for it from someone else, otherwise finding it in the wild, cheats would be required.