Do you mean the airlock where you can bring your rover into Astrape? If so, you first have to go to where the airlock is and cycle it so you have access to your rover. Then, you have to go to the rover and click "drive into airlock". Once that is done, you have to get out of the rover and cycle the airlock again. After you do this, the rover will be inside Astrape, and you won't have to go outside to replenish your air.
I hope this helps!