Umm...right behind the castle. But if you need more specifics: Across from the exit to Fairytale Hall. If you are standing at the carousel looking at the castle it will be on the right side. It's next to a gift shop. There are water fountains. And a Cinderella statue. And a crown painted on the wall. If you walk up main street and turn left towards Liberty Square and then go over the bridge and turn right and walk up the walkway towards the castle once you get to the top of the walkway the fountain will be on your left. If you get to Philharmagic you've gone too far. If you are walking from Peter Pan and you are headed towards the castle you will walk past Philharmagic on your right and the fountain will be just after that on your right. If you are coming from Pooh you will walk towards the carousel, walk left around the carousel, then keep going straight into the wall in front of you and you will probably be at the fountain. The fountain is not near Merida's meet and greet. The peoplemover will not give you a view of the fountain.
If you're in Frontierland, Adventureland, or Tomorrowland you are not anywhere near the fountain. If you are in EPCOT then you are way off.