you can rent a 4wd, or just get someone in a tourist office to take you there. you can also get a taxi that will cost you maybe 1500 or 2000UM to the road point to atar to terjit. maybe after you can alk a few kms or just pay the taxi to take you there. road is hard though.
here it comes again, big machine this one. hehehe, it went everywhere in mauritania. people went crazy when we arrived places with such a small car. usualy places are only accessible with 4wd cars.