It's in the orc village.
I beleive the kafra in Prontera will warp you there, otherwise just head west from prontera until you reach the salt flats with Ambernites on it, and then turn south.
World MapThe dungeon is accessible in the hut in the north west (to enter the northern portion) and by the hut in the middle of the map (to enter the southern area with the zenorcs).