Go to www.youtube.com
Type in the name of the song.
Click on the video.
Highlight the URL and copy (cmd c)
Go to www.dirpy.com
Paste the link (cmd v) click dirpy
Click record to mp3
iTunes will open, pause it and close iTunes (command q)
Import the song from downloads into your music library
Click your file
Your song is in your library, it will start playing when you click it.
Hold down command and click and drag your song from your music library onto your background slide. When you click your background, your song will play with it.
The creator of this guide has not included tools