It's deep in the heart of Africa. Start on the coast roughly halfway between Marrakesh and Algiers, and go due south. Most of the area is impassible. There is a road leading there if one has been built which will make it easy to follow. The road actually begins farther east, right at a river mouth, runs south through a mountain pass, turns west for a bit, before turning south and running straight down. Look all the way down on the southern edge of the map deep in Africa. You should see a semicircle river in the desert. That's Timbuktu.