When you enter the room where Sully is associating with the crooks, you will first have to kill all the guards. Afterwards you will have to solve a puzzle with the statues in the center of the room. Press Select to enter Sir Francis Drake's diary. You will see that the four statues are next to either north, south, east, or west. Then, exit the diary. The center of the room has a compass painted on the stone. You must turn the statue to the same direction indicated on the compass. Once completed, a secret door will open.