you can find creselia on moonlight island but i dont know about darkrai.
Yes creselia is is in Pokemon Platinum.
You will have to trade it from Diamond, Pearl or Platinum to get one.
Darkrai
No.
Nope
you get event Pokemon then go cacth creselia and go to canalave city walk to the harbor inn and stay there for a night the you will have a dream you will be on a island and walk up to a puddle and darkrai will be there
action replay
Darkrai.
Ok the answer is yes, but only throught the darkrai event
Darkrai is a Pokemon from Generation IV, and Pokemon ruby is a game from Generation III , so you can only get darkrai in Pokemon Platinum, Diamond or Pearl
I am not sure how to catch darkrai or creselia but to obtain Palkia or Dialga you have to go to Spear Pillar in mount Cornet.
You dont catch Darkrai in Pokemon Pearl. You can catch it in Pokemon Platinum at level 40.