Hi gksullivan,
Video clips that you've taken with an iPhone are compatible with the Mac, so you shouldn't need to convert anything. If you are downloading video from iCloud to a folder on your Mac, and then importing that video into iMovie, are you getting an error message?
The easiest way to get the video clips you've recorded on your iPhone into your Mac is to import the video files into a folder using Image Capture, or import the video clips directly into the iMovie application. You can also AirDrop video clips from your iPhone to the computer. I just did this from my iPhone and a video named FullSizeRender.mov was transferred.
iMovie (2013): Import from iPad, iPhone, or iPod touch
https://support.apple.com/kb/PH14672?locale=en_US&viewlocale=en_US
iMovie User Guide
http://help.apple.com/imovie/mac/10.0/#mov142f39686
Image Capture: Transfer images and other items from your device
https://support.apple.com/kb/PH17894?locale=en_US
Share content with AirDrop from your iPhone, iPad, or iPod touch - Apple Support
https://support.apple.com/en-us/HT204144
Take care,