You have to go in the house near the boat that sails away to Iron Island, and you'll see that the mentally insane kid who can't keep still has fallen dead- I mean ill.
Then you have to go out to the dock and sail to Fullmoon Island with the sailor guy. It's a lovely place. It really is. I'd camp there if I could. Go into the forest bit and click on Cresselia and it'll disappear. Pick up the Lunar Wing on the floor and take it back to the kid. Once you give it to him, he'll wake up. They don't give you anything in return. How rude! HRRRGRRR... *Eats*
Then track Cressy down and catch it (not with the Masterball, because Cressy is easy).