how to convert an MP4 movie to MP 3 on my imac

What about Garageband?

There's many utilities out there that do this kind of thing, though with some it is a bit more involved than just pressing a single button. I have at least half a dozen that will do it but some are older utilities (I use Mavericks) and some I downloaded 10 years ago. VLC may do it but you will have to learn a few settings. Some you may need to do two steps, demux the streams first, then use another tool to do the conversion from mp4 audio to mp3 if you still want that.

mp3 is not a video format. It is an audio format.

If the mp4 doesn't play on your Mac, VLC might be able to play it.

If VLC can't play the mp4, then it is likely corrupt and unplayable on anything.

VLC contains most of the known codecs in the universe.

Try using VLC to play the video.

Why do you want to convert a video to an audio file? You won't be able to watch much of anything.

Have you tried using iTunes?