So I have just caught all 3 legendary deer Pokemon in omega ruby and now just have cresselia to catch. I have been waiting a long time for crsselia to spawn even before I got the legendary deer trio. I am getting really frustrated and could someone please tell me the easiest way to get crescent isle to spawn and the mailbox Icon that tells you notifications isn't there in the pokenav for me and I have enabled street pass. Could someone please tell me if there is a easier way then using the pokenav and if there isn't how to get the mailbox icon to appear. Please help!
As you have mentioned, Cresselia appears on a rare Mirage Spot called Crescent Isle.
Unfortunately, you don't have any control over which Mirage Spot you get. So, as mentioned by pinckerman in his comment, the best way to find it is to go online through the PSS.
As stated on Serebii.net, you will get the mirage islands of the players you encounter through StreetPass/SpotPass if they have visited their daily Mirage Spot so do visit yours before connecting so you can share it as well.
Once you get to the island, you can save your game and it will still be available if you decide to try and capture Cresselia another day or if you want to soft reset for it, which often takes a few days.