to create buildings floor by floor, you need to make a concrete floor plate. first make a floor plate the size you want it then raise it up about 0.3 meters. in the middle of the floor plate create a building "core" (draw a square) and raise this about 3.2 meters on top of the concrete floor plate. then draw squares or circles around the perimeter of the building for supports, and raise these 3.2 meters. put up your cladding, and add decks where you want them.
then when your done that, select everything, and multiply it, so you make a tower. once your done adding the floors, add some roof detail, and your done!
p.s. to multiply floors all at once, type in *then the number of floors you want to multiply right after you multiply the first floor.