I'm having the same issue myself. I posted the question on this forum http://goprouser.freeforums.org/cineform-mov-format-and-imovie-compatability-t21 163.html
Here's the same post incase it helps:
I've search multiple forums and haven't had any luck fixing my issue so it's time to post. I'm running os X Yosemite, and iMovie 10 (it's all up to date). I've been using the go pro studio editing software so far and wanted to switch over to iMovie which is when I've started getting all the problems. My gopro is the hero 3+ black and the footage is at 1080 60fps
1. imovie will open the mp4 file straight from the camera, but my laptop doesn't have to power to actually work with it in a usable fashion (choppy).
2. I converted the file using go pro studio (so now it's an MOV file) and iMovie grays out the file.
3. After much searching I learned that the MOV is in a cineform studio format (for gopro software) and that I need the cuneiform codec pack to use these files. I installed the pack and restarted.
4. iMovie still won't recognize the files. Quick time will now open the file, but it runs conversion first and then the file is playable. Upon closing quicktime it asked me if I want to save the converted file. This converted file WILL open in iMovie and play the clip properly (yay!).
5. The cineform mov file still won't open after this.
So if I want to use iMovie the process would be - load file onto computer - convert to mov using gopro studio - open converted file in quick time to convert again then save - open in iMovie. This process just doesn't seem right and takes up way too much hd space.
It's relieving to find out someone else is having this issue as well. Hopefully we can figure it out
