The resistance hideout is at the Dark Alley. Garlef (at the Tavern) will know the secret knock. You have to beat him in rock-paper-scissors game.
This is the low-spoiler guide for "Getting into the resistance hideout" puzzle in Chaos on Deponia. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.