The Culling of Stratholme is located in the Caverns of Time, which is in Tanaris (Kalimdor continent). You can teleport there quickly by taking the portal in Dalaran. Additionally, speak to the dragon at the entrance to get a free ride into the caverns. The portal to Culling of Stratholme is in the southeast inside the Caverns of Time (just to the left of the Black Morass). There are human NPCs and buildings outside of the portal.