Assuming you have completed the 4 sub-quests, the entrance to the Hall of Echoes is far below the talking tree that had the riddle for you, a little south of Keara's flying fortress entrance. Head down to the beach where there are bunch of dragon skeletons, and go to the back, to a cave-like entrance.
There is no way back (until the add-on, anyway), so make potions or upgrade enchantments, etc, before entering.