Follow the corridor until you get to a dead end, and are facing a wall bordering the sealed room (in the north west corner of the dungeon walk south). That is the wall that can be removed, and which has the pressure plate on it.
I just check in-game, and the description for the hatch when you are outside the Forgotten Outpost is called Jackson's Dungeon. I noticed that when I went though that part of the game, but forgot about it (and the D2:ED walkthrough / DKS FAQ don't use that name).
