Go to the menu entry "Building" "Add Floor" and the "Add Floor" dialog will open (2.2). In the upper field you can indicate a unique name for the floor and give the floor an altitude in meters. This altitude should conform to the relations in the real building.
In the lower field of the dialog you may select a graphics file, that means a map, that represents the floor. Select the right one and close the dialog with "Finish".
Feel free to add additional floors to your building. With "View" "Floors" you can select the floor to be shown in the main screen. With the magnifying glasses in the tool bar you can zoom in and out of the map. Predefined values of the scale can be reached in the "View"
"Scale" menu.