Standing near the cave-like entrance, you need to read the 'Note From Maxos' in your quest item inventory in order to remove the glass/ice blocking the way.
Just to make sure you are at the right spot, the entrance is far below the talking tree that asked you some riddles, a little south of Keara's flying fortress entrance, east of the Red Hammer Tribe teleporter shrine. Look for the beach there with a few dragon skeletons further inland; the entrance is at the back.
