You need to help the boy who has a nightmare in valencia island first. You'll need a Munna to find out the cause of his nightmare. Then you'll have to interact with an occult NPC in Valencia Island PC. Then Cresselia will appear in Lunar Summit.
Also does anyone know where the move relearner is?
There's one at lavender town that requires mushrooms
There's also another at Kinkan island that is money only.
How can I evolve my eevee*?
Okay, Beta 12 DEMO is out. Ends at Lavender Town