Hi
I have converted a lot of music from youtube to mp3 . There are a lot of on line applications that will do the conversion for you.Just search "youtube to mp3" and pick one. I prefer "listentoyoutube.com". I had the issue you described too.What has worked for me (don't ask me why that it,I haven't got a clue but...) was to open the application in Safari and the youtube in Firefox or the other way around ,but not both under the same browser. So here how I do it. In Firefox open youtube ,play the video you want,and highlight the downloads window above,it will turn highlighted blue and then click "command" and " C " on your keyboard . Minimize Firefox and open Safari and go to "listentoyoutube.com" once there on that page click in the conversion window and on the keyboard click "command " and "V" .The URL of your chosen video will appear in the conversion window. Follow the instruction on how to convert it's simple and your video will be converted in under a minute and then it downloads in your iTunes library as mp3...Try that and see. I am not computer savvy either..
Regards
George