To make this work you actually have to drop the files on the "unmatched files" folder node, which is small, and on Windows XP, at least, gives you no indication/highlighting that you're over the top of a location that you can drop things on. Highlighting seems to work on Windows 7, FWIW.

Since you can't forcibly drag stuff into Clusters, I don't believe, perhaps a drag to anywhere in the left-hand pane should work?