I Copied the AVCHD.RADPlug directory and all the content within from iMovie 09 8.0.3 or greater and it worked for me as well.
I have a Panasonic ZS3 and recorded in the AVCHD lite format. Days of trying every format setting and every other manipulation and nothing worked until this.
I used a friends mac that had iMovie 09 and copied the contents. Having spent 11 years a tech support person I’m pretty confident with this fix.