The software is just a video downloader, meaning that you can download videos to one of the supported video formats. There is no converter, so you won't have the possibility to save MP3. The file format is the one given by YouTube.
Luckily, you could use another application to convert the file to MP3 or switch to other downloader that has conversion capabilities.
Use the following link to reach the search results page with apps that can convert the downloaded videos to MP3.
Search results: http://software.informer.com/search/youtube+video+download