For aircraft that can dump fuel, mainly longhaul, the fuel is dumped straight out of the tanks via a jettison pipe. The outlet is usually near the wing tip. The jettison fuel pipe situated in the fuel tanks is set at a certain position to prevent total emptying of the tanks. I think Air Traffic try and place the aircraft over the sea if nearby but otherwise overland.
Of course it all depends on the nature of the problem requiring the aircraft to land in the first place!!