There's a known bug: sometime you sit down and nothing happens... that's how I missed it.
Reload a previous save game and try again.
The bug even occurs if you fight the Tunnel Snakes gang outside the door. If you wish to get the good karma, talk to them instead.
If you're on a long playthrought, you can always reloald an old savegame, ignore the guys and run to GOAT: it's works.