By working your way down the navigation tree structure, make sure the floor you wish to open is visible in the navigation tree (see image below).
Click the floor you wish to open (see the first image below). Note that the floor you click will open. When a floor opens, so does its floor plan card (see the second image below). Via the floor plan card you can work with a floor. For more information on how to do so, see chapter 9 in the manual for Buildings. You will find it here.
