If you have modeled the structure using the building modeller and have assigned the superimposed loads (weight of slab, live loads etc.) while creating the frame elements, the program will automatically consider them while performing the analysis. You don't have to define them separately in the load cases tab. Of course, this is applicable only in the case of gravity loads (G, G' and Q). Analysis types such as eigen value analysis, static analysis etc. will run with just the self weight even if no other loads are included.