You should be able to simply click on the two headed arrow just to the right of the check box and drag the script's name in the the script manager into this folder while the folder is open. It can be a bit touchy as you may have to drag a bit to the side to get it to indent (which then places it inside the open folder).
How wonderfully simple. I was dragging up, but not to the right. That's the trick. Thank you so much.