You can't assemble anything if you don't have any parts in your assembly. First you have to add parts from your parts library.
It's the same in ST7. Assemble is a mate tool which is meaningless without something to mate.
@cs2kplus What may have tripped you up in ST5 vs ST7, is that in ST5 the user has to kind of know that to start an Assembly you go to the Parts Library tab and start dragging in parts. We found this to be a hiccup for new users. So in ST7 we added an "Insert Components" button (right next to Create in Place and Assemble) which pulls out the Parts Library tab for you and makes this really obvious.
So probably going back to ST5 you were missing this button and that is what threw you for a loop!