It's been a while since I was in there, but I think you're in quests for Watson and when you finish he sends you to Katz's lab. If that's not totally right, someone correct me please. The door is across the roof from Watson past the cell where the frog was. However, the door is just the start of another quest series, and it's a story line series. Good luck.
how do you get the door to unlock?what is the combination? you have to click on 3 placque to open which ones?
Watson tells you to fight Firegut to get the combination. After you defeat Firegut, he tells you to "look up" as you leave his area. The "combination" is on the wall "above his door" as you are leaving.
1. Fight Firegut. Get first combination (sun, bird, tree, beetle). Go back to Watson and open first door. (This will stay open as long as you don't leave the dungeon.) To open the door, there are 4 plaques hanging on the walls alongside the first door. (NOTE: They are very hard to see on my computer. I have to walk right up to each one to see what image is on each plaque.) Press "x" in front of each plaque following the first combination you just got.
2. Second door is locked. Watson says to go fight Clanker. Get the SECOND combination from Clanker. Return to Watson. He says to open the second door. Use the 4 plaques again, with the SECOND combination from Clanker (tree, sun, bird, beetle).