.mkv is a container format, not a codec. Much like you can have a .mov container with Sorensen codec video and IMA audio, or an .avi container with DivX codec video and MP3 audio.
If you're familiar with using Terminal and command line stuff, you can grab a package of MKVtoolnix here:
http://www.bunkus.org/videotools/mkv...ds.html#macosx
You'll want to demux (split) the .mkv file into component audio and video streams, and then mux (join) them back into another container, like .mov or .avi, depending on what you want to do with it.
Chances are the video stream will be in DivX codec, but you won't really know until you get it demuxed.