The USB should work - always have it connected to mains power as well.
Having said that, I have a similar camera but always use the SD card reader in my macbook.
I find it easier to drop all the video files into a folder on my desktop.
I can then preview them (highlight and press space bar), pick the ones I want to use and import them into iMovie grouped into separate events.
You do not necessarily want everything off your camera in iMovie all at once.