Once you have completed the first level of "The Endless Paths of Od Nua" and you have left the dungeon through a cellar door, clicking on the cellar opens the map where you can pick the dungeon level you'd like to go to.
However, opening the map first displays an empty map. The only unlocked dungeon is on top of the screen.
I don't think that it was intended that I first need to scroll around.
Compare screenshots and savegame.