I ran the program on a Mac (Mountain Lion 10.8.2) and downloaded a Video from Youtube. Actually it was saved without filename, just the Suffix (.flv, .mp4). This means that Finder won't show the files, because on Macs, every file which starts with a dot is invisible. So I had to rename it via Terminal from ".flv" to "Name.flv". I've included a screenshot of the program after downloading.