I may be dense, but I can't figure out how to indicate the "second directory" -- the one to be compared against. When I selet browse, I get another window, then navigate to the folder i ant and click open, but the directory name doesn't transfer, I'm pretty sure I.m missing something obvious...
Try dragging the folder into the Find in area.
That or double-click "Add folder to search" in the list of folders.
Note that the duplicate finder just looks for two copies of the same file anywhere below the specified folder(s). It does not need two folders; it can work with one, two, three, etc. The results will not be restricted to either folder, so if the same file is in one folder twice, it'll be listed, even if it isn't in the other folder at all.