Like someone suggested earlier, you have to transcode it to the Apple Intermediate Codec so that it can edit natively within iMovie. MP4 is not a recommended format for editing.
If you have Final Cut Pro, it's best to edit transcode the footage to ProRes. Unfortunately, Final Cut Express doesn't support ProRes editing. It's only able to decode ProRes files.
With that said, the video quality is amazing on the Mark II. Assuming that you have a fast lens, it's even more incredible for low-light shooting. Just wish you had manual control over aperture and shutter speeds...Oh and ISO speed as well. ISO speed is important for low-light, since the higher that you get, the noisier the video gets! Wish we at least had that option to set it manually! I'm waiting for a firmware update! C'mon Canon! You can do it! 😀