Easiest and best way to convert a MKV file to a format compatible with iMovie is by using Handbrake. Assuming Handbrake is installed (otherwise : brew cask install handbrake), follow these minimal steps.
- Run Handbrake (obviously).
- Import your MKV file using the « Open Source » button.
- Select MP4 as the output format.
- Choose destination using the « Browse » button.
- Start conversion process by clicking on « Start » button.
After Handbrake finished converting the MKV file to MP4, you should be able to import it in iMovie and start editing the video.
