Hello!
You can go to www.youtube-mp3.org (http://www.youtube-mp3.org) and place the URL into the text box to download the desired song.
Hmm, try this site then: www.youtubesoundripper.com/ (http://www.youtubesoundripper.com/)
If this doesn't work, then I suggest you simply Google for a "Youtube to MP3" site yourself and find one that works best for you.