In general, your aim here is to fully power, fill and free the giant airship that you find at one end of the central giant chamber. Once it's moving, it'll go all the way around back to where you started from, and knock down the building you begin near. This will get you the symbol you're looking for. So don't spend a lot of time trying to get into that first building or getting out onto the dock area. That all happens at the end :)

The Myst 3: Exile Walkthrough