First, you need the National Dex. Go to Canalave City and enter the Sailor's house. You will notice that his son is sick and is having nightmares of a specific Pokemon also known as Darkrai. Go to the sailor and he will take you to Full Moon Island, this is where Cresselia resides. Once there, adress it, You will see a picture of it, then it will flee. When it does it will leave the lunar wing behind. Take it to the sailor's son and he will be healed. The sailor will reward you afterwards. Cresselia will be roaming the sinnoh region, like mesprit. the easiest way to catch it is by using mean look on it. Then it will no-longer be able to flee. then weaken it, and launch ultra, timer(and at night dusk balls)at it. This should get you cresselia.