Go to fullmoon island & catch cressselia.
Chat with our AI personalities
In the house next to the sailor who takes you to Iron Island.
Talk to the man that brings you to Iron Island, he'll say that his son is ill, go to the his house and talk to the kind and the mother, than go back to the man and he'll bring you to the insland where you can find Cresselia, when she flyes away you'll find the feather to cure the kid.
Once you beat the Elite Four and get your sinnoh Dex. Go to Canalave City, and go into the sailors house, talk to the sick kid and his mom. Then, go outside and talk to the sailor who took you to Iron Island. Talk to him and he will take you to Full Moon island. Talk to cresselia. It will run, but track it with the marking map. Just use the same method that you used with Mesprit. If this doesn't help you, their are guides on the Internet. (Youtube) GOOD LUCK!
Well, you talk to the kid, then talk to his mom, then go to the sailor outside who says his son his having a really huge nightmare (not exactly but you get the point), then he'll take you to this place called Fullmoon Island where the pokémon Creselia lives. When you get to Creselia, she flies away and leaves the lunar wing behind. Take the lunar wing to the kid and he'll wake up (and if you want to get Creselia you'll have to look on the marking map like you did for Mesprit).
Talk to the sailor. Do what he says go to crecelia press A then get the pokeball give it to the boy.