first beat the league then head to the sailors boat he says his son is ill and only a lunar wing can cure him he takes you to an island wher you encounter cresselia as a Sprite talk to it and it fly's away leaving the lunar wing give it to the sick child then you can catch cresselia the same way as mesperit and if you don't have the marking map go to jublife city and go to the poketch company and talk to a guy who will give it to you.
you cant catch it anymore
Cresselia is found in Pokemon Platinum as a roaming Pokemon. After encountering Cresselia for the first time, one can follow its location by checking their Pokedex entry for the Pokemon. Cresselia can potentially appear in any patch of tall grass.
visit full moon island and look for it in the wild
It's possible but you may not be able to catch a cresselia afterwards. Problem is, I haven't figured out how.
Yes, to catch Giritina, Cresselia, and Mesprit. In Pokemon Platinum, you can catch Zapdos, Moltres, and Articuno also. :)
cresselia is a roaming Pokemon, to catch it you have to follow your marking map on poketch this way it will help you find it or some other roaming Pokemon. hope this helped :) PS: the roaming Pokemon are... Zapdos, Articuno, Moltres, Mesprit, and Cresselia
you have to follow her to catch her like with mesprit. just follow your poke etch
chase it with the first team member that is faster than it and teach the mean look so the Cresselia can't run
you find cresselia on full moon island
Uxie; Mesprit; Azelf; Dialga; Palkia; Heatran; Regigigas; Giratina; Cresselia; Phione; Manaphy; Darkrai; Shaymin; Arceus
This is the same way from Diamond and Pearl.
You catch Cresselia