Magix Movie Edit Pro 15 Plus Screen Capture help?

Helpp!! I have to get video from Youtube and put it on Magix to make a space video for school. When I have the little box on top of the Youtube video, it freezes and skips. Ugh! They just look like still picture s and skipping and stuff!! Ahh!! HELPP!! PLEASEE!!! It also says something about bLack screens being recorded and video overlay-what the heck is that? Thhannkkk yooouu!!!!!!