Well, that explains a lot. Most of our instructions assume you're viewing your music using the regular library. Music Explorer isn't designed for that (although you should make a bug report about that error you're getting). See, this would have been cleared up very quickly with a screenshot (and yes, you post images by url, using imgur or other services).
To use drag & drop the way you're asking for, you will need to be in your library with the column browser or thumbnail browser active (see the arrange panels settings next to the search box). I believe it's activated from tags (2).
I strongly recommend you take some time to explore different library views, using the menu at the top right of the main panel (from Music Library, not Music Explorer). Likewise, check out all the options in the arrange panels menu.