To open the MP4 file you want to edit in iMovie please follow these steps.
- open iMovie
- At the top of the iMovie screen click on the Media tab.
- Click on File at the top of the screen and select import media.
Import the video file you saved to desktop or known location.
Edit the file to your liking.
When finished editing you can save the file to your desktop by clicking file at the top of screen and selecting share>file.
Save the file where you can find it.