You add slab edges by selecting horizontal edges of floors. You can also select model lines.
Where is it?
Build panel
Floor drop-down
Structure panel
Floor drop-down
You can also click model lines. As you click edges, Revit treats this as one continuous slab edge. If the slab edge segments meet at corners, they miter.
Placement panel
Select panel
Modify. 
Slab added to floor
Tips