First of all, are you on Lion or newer? MC does not support video playback on Snow Leopard, nor does it work if you have an older Mac that can't support OpenGL 3.2:
http://wiki.jriver.com/index.php/OSX_System_RequirementsAssuming that you meet these requirements, MC will treat (and try to play) any file as a video file if the special [Media Type]
Library Field set to Video. MC should auto-detect the proper [Media Type] for all audio and video file types you import, though occasionally for very unusually (typically broken in some way) it will mis-detect a file and assign the incorrect [Media Type]. If this happens to a few files, you can fix them using the Tag
Action Window.
If you're somehow getting ALL video files (regardless of type or source) imported as audio, and your computer meets the requirements for video playback described above, we'll need more information. How exactly are you importing these files?