Hmmm... that works here. I just created a new set (empty) then dragged a font from the bottom-left pane to the the new set folder in the top-left pane, and it added it just fine.
I have had to trash the whole folder full of suitcace prefs and sets on occasion, but it seems like it's settled down with that since about 10.2.
But wait-- it won't let me add System fonts to a set that way. It only lets me add my other fonts that I've purchased and keep seperate from the system fonts. Are you trying to add a System font? Perhaps that's a limitation. I would suppose they're trying to idiot-proof the program, so as to keep you from turning off vital system fonts.
CV