Make sure you activate the MPEG-4 codec in the program. In Movie Edit Pro 14+ this can be done under the "Help" menu, under "Activate additional" functions. Request an MPEG-4 activation online, activate the codec and try importing the files again.
I had the same problem. I paid the $5.00 for the activation and it didnt help. It appears that it activates your ability to EXPORT mp4. It did not allow me to open/import my mp4/HD videos. I did find something that said MEP 15 will not open HD with horizontal video lines more than 768.