I presume that you are using the trial version. If so, it has limited features. AFAIK, reading certain file types, like mp4 requires a Codec that Magix has to pay for, so they don't include it in the trial version.
What is it that you want to do with your videos? Make quick and dirty edits, mostly with the templates? Or do you really want to edit videos with sound in much more detail. If so, then look at Movie Edit Pro Plus or Premium (not the basic version). If you are taking action videos with the GoPro, then you will probably need really good stabilization which is in the Premium version with Mercalli 4.
I had a full blown editor, it decided it couldn't read the mp4 files, even after doing it for two years. I dumped it. I will have to see where my budget is, for a new editor.