How do you get a cresselia?

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!