I sent my files from my PC to a MAC in mp4 format. However, I cannot open the files with quicktime (only with VLC) and I cannot figure out how to edit the videos using iMovie because iMovie will not let my import them!
You could use Mpeg Streamclip to convert them to something editable.
What dimensions do those videos have and what frame rate, and are you sure they are in mp4 format. Usually QT should read them.
Maybe installing Perian might help.