I think that the easiest way is to take the X9 bus (not 109, as X9 is an "express" line) from TXL to Bahnhof Zoo. Then, walk about 10 feet to the stop for Bus # 200. Exit the #200 bus at Potsdammer Platz, and you are about 100 meters from the Marriott.

The beauty of this route is that it is all on the surface. No stairs to negotiate, no elevators or escalators to search for.