Sorry, that was a little ambiguous wasn't it.
I can drag .avi's and .mpg's into the library (or just right click and select "send to library") and can even do .mp4's, but in the library window, it does not show any .mkv files. That is, if I view a folder that contains 5 avi's and 5 mkv's, it only shows the 5 avi's. I've been unable to find any option to change this.
Any help would be appreciated :-)