You need to get all 4 small room doors open, and then pull the lever in the central area to close them and open the treasure room door. Using the teleporter pad to leave the house and then return should reset the doors and levers, if required.
Specifically, pull the lever in the NW room down first, then the one in the SE room and then the lever in the central room. The levers in the SW and NE rooms stay up.
