I just reached level 50 and was summoned by a horse guy but I do not remember where he is? Please can someone help??
Also I went to see Merle and he says that I should go and see him has needs to tell me about Celestia. That is all he says, so how do I get him to tell me about Celestia?
The horse guy was Diego. He is on Unicorn Way. He will explain "critical rating" and give you a hat to get started.
As far as "going to Celestia", Zeke said that "Players who have reached level 48 and completed the Dragonspyre quest "The Final Countdown" should speak to Merle Ambrose for more information about Celestia."
Did you complete the Dragonspyre quest yet? If not, after you complete "The Final Countdown", then Merle Ambrose will tell you about Celestia.
(Just a side note: you don't have to go see Prof. Halstrom in the Storm School. He gives you a quest to do a very hard warehouse in Marleybone. This was a "required" quest at one time to get into Celestia, but KI changed it and you don't have to do it, unless you want to.)