In order to reach the designated area quickly, you need to fast travel to doctor Earnhardt's place. Next to the estate, there is a hang-glider that you can use to reach your destination quickly. You need to be careful, though, and make sure that you aren't spotted or don't land too close.
The cell key is held by the camp operator, who is marked with a star. In order to get lower, you can simply jump down in the area shown in the screenshot. You need, however, to be careful about guards underneath.
You can approach the operator's tower from the left. There is a makeshift podium there, which you can walk up to reach him and take him out quietly. The nearby guards Shouldn't notice anything. Collect the key from the corpse and go ahead.
The way to the cell is well guarded. Before you take every step, you should look around carefully for any guards and eliminate them quietly. Another method is to sound the alarm purposefully and wait behind a cover at the entrance to the cave until the enemies come out.
Open the or with the key and you will be standing in a cave with a projector. Go further into the tunnel (the screenshot), where you will be stunned.