You need an Event or get it from a trade.
______________________________________________
but there aren't anymore events for Pokemon Diamond are there?
Yes, Darkrai's number in the Pokedex is #491. It is identified as the Pitch-Black Pokemon.
491
no
Aerodactyl
486 is regigigas and 489 is darkrai
Marriland - "http://pokemon.marriland.com/diamond_pearl/pokedex/darkrai" Serebii - "http://www.serebii.net/pokedex-dp/491.shtml" Smogon - "http://www.smogon.com/dp/darkrai" + gives you strategy Bulbapedia - "http://bulbapedia.bulbagarden.net/wiki/darkrai_(pokemon)/"
Obviously you need it to complete the national, otherwise darkrai will be missing:) not needed for regional though.
Without seeing cresselia, you must go to bed during the night (approxiamately 10pm) and he will appear in a dream. that is how you get him on your pokedex.
darkrai i think. You can get darkrai at a Nintendo special event or by using a cheat which i don't know. P.S. I've got a darkrai, by Nintendo event!
If you really think that someone is going to answer that, dream on. I will tell you that there are 490 Pokemon in the Pokedex, but that excludes Darkrai and some others.
Darkrai is #491 in the national pokedex, and it is a Dark type Pokemon.
you have to complete the national pokedex then sailor eldrich will take you to the other side of full moon island and that is where darkrai is on platinum.