The Album Artwork manager is view dependent, not selection dependent. It is meant to find problem files (according to the filter settings) among a large group of files displayed in a tab, not to individually manage artwork for albums or tracks.
Your best bet to get album art would be to select the file or files you want to get art for, right click, edit, and then manage the art from there or to use the Downloader tool as you said you were doing in the other thread.