I use Movie Edit Pro 14. I tried to load a *.mp4 video and received the message that it cannot be opened in Magix Movie Edit Pro 14. I purchased the upgrade for *.mp4 but I think the upgrade may not have installed properly.
Or you are trying to import a HD mp4 which has a HD codec that version 14 does not support.
You can analyse the video using GSpot or MediaInfo to find out which codec is required (mp4 is a container file and can have any codec inside). GSpot will also tell you if the codec is installed and the video can be rendered (don't know about MediaInfo)