I was having the EXACT same issue as joyful.vegan, using PhotoBooth and then iMovie. I finally figured it out after reading through the posts here.
I have iMovie version 10.0.8 using Yosemite OS
Once you've imported your PhotoBooth footage into your iMovie and you're working on your movie, dragging clips into your timeline, do this:
1. select the footage IN YOUR TIMELINE, that you want to flip (it will be outlined in YELLOW once selected)
2. go up to the tools and choose the "video and audio effects" tool. (it looks like 3 overlapping circles and is in between the speedometer icon and the "circle i" icon.
3. click on that "video and audio effects" tool
4. it will bring up "Video effect" on the left side of screen and the "Audio Effect" on the right side of the screen.
5. From there, click on the "Video Effect" box to view choices and then choose the "Flipped" option