this is what i have done in the past. this is using your action window.this would be in the bottom left of the application. if Action Window in minimized, then it the up arrow just to the right of it.
if action window is up, then hit Tags
first thing i would do is to make sure media type is listed.
if not, then just right click to pull menu, hover over "also Show" and select Media Type (for Video)
and since youre there, add media sub type (for movie)
once those show up, the rest is easy.
select all the files you want to be made Media Type - Video.
once select is done, head over to Tags (bottom left) and fill in the blank for media sub type.
then you can do the same for Media Sub Type
to make those permanent, you need to make sure you have "Update Tags when file info changes" checked
that you can find in Options - General
(i'm not sure it actually tags some video files, but it will add a xml file which will record tag info for later importing)
otherwise, if you have to import again, u'd have to repeat those steps
Hope this helps you a little.
i can't say i know everything about this software, but i do love my library crisp and clean
Scott