Happnes if you have the panel orientation set to horizontal and placed on the bottom of the screen. Just place xfmedia right on top of the panel and try to add a file/directory/url. It won't be immediately obvious that you can press 'add selected' or 'add + close'.
sorry, but that's a window manager window placement issue; nothing to do with xfmedia. i don't think it's wise to force placement to, say, the screen center, when expected behavior is generally that these types of dialog boxes are placed according to the WM's rules (usually centered over the app itself).