Just guessing... I just asked google and it seems that .cpi files is one of the newer AVCHD videos based on MPEG-4 format. So as a start you may have success simply by renaming your .cpi file to .mpg and see if MEP accepts them. If not I'd suggest searching for a video file format converter (I use one called 'Super') until Magix catches up with the ever changing video technologies. Camera manufacturers seem to delight in creating their own 'industry standards'! :-(
Like Carl I am guessing you have a Sony HXR-MC50 HD camera or similar, in which case the cpi files are not the video but clip information files which the camera uses to track the movie and image files.
The movies, I think, are stored as mts files. If they are mts fles then they wil import directly into MEP.
Also you may have some software with the camera which can import or convert the files.