First, go to Canalave City (Cresselia is in both Diamond and pearl versions). Look for the
boy in Sailor Eldritch's house. Talk to the boy in bed and he will be saying something in
his sleep, something about Darkrai. 

Talk to the mother next to the boy in bed. Next go outside of the house and talk to the
sailor next to the ship that takes you to Iron Island. He will say something about a
Crecent Wing. Then he takes you to an island. On the island, there should be a clearing,
go into it. Then you should see a pink-yellow pokemon. 

Talk to it, and it will appear on your screen and will be added to your pokedex. then it
will disappear. Its kindda like catching Mesprit. 

If you did not get the pokemon leauge or get all the pokemon in the Shinoh Pokedex, you
cannot catch Cresselia. Now if you did not get the pokefinder application, it will be much
more difficult to catch Cresselia. 

This is how you get the app, you have to go to the Poketech company in Jublife City, talk
to theman next to the desk on the FIRST FLOOR, He gives you like 6 or 7 apps, the
pokefinder app is the second or third one he gives you, YOU CAN GET THEM ALL IN THE SAME
DAY! Now, good luck catching Cresselia, oh and one more thing, i think altering between
route 205 and the Valley Windworks isthe best way to use. And you should stock up on MANY
quickballs. Happy hunting!