This extension still works for me. I go to Youtube, I look up a song, I click the extension button, it opens a new page and gets the mp3 DL link ready. When ready, I click the DL link. It opens a new ad window. I close that new ad window, and the mp3 DLs like it's supposed to.
Sometimes it won't DL the mp3, or will say the link is bad. In those cases, close the DL page and try the extension button again. If it keeps saying the link is bad, there's nothing you can do except try a different one, or accept that you won't be able to DL the mp3.