Check the Aldlyn city board to see if you have done the Thremten festival already or not. If not, then that's obviously what you're missing.
Did you have Mirel in your party when you spoke to the 6 people related to the hot spring? https://renryuu.miraheze.org/wiki/FAQ#Secret_hot_spring
Have you pressed enter when you where on the right spot? The entry doesn't work automatically like other teleports, but it needs to be triggered by the player, since it's a "hidden" path.
