The first one, sewers is accessed by doing the first mission for the hobo camp and being tasked to go into the sewers. The second dungeon is required for the 3rd mission with Dr. G who if you haven't met you interact with the person sprinting in the courtyard in sector B but you should be level 10 (it recommends 8 but it really is incredibly hard.) I would also recommend doing the lipstick mission first.