Klaus1 mades some excellent suggestions.  The Plex suggestion solved the issue for the poster.

t could be a ‘difficult’ form of Matroska file. FYI Matroska is just a container like MOV or AVI, and some mkv have the video encoded with Real Video Codec (RV9 RV10). Perian+QT can’t decode this, but VLC will play .mkv files just fine on OS X, as long as they don’t use H.264!

It’s a bit of a lottery until Matroska decide to support the Apple platform. I have found this:

http://mkv4mac.free.fr/index.php?lang=en&p=lec

which may help.

But there may be another workaround: 1. Get Info the mkv file
2. Change to extension to wmv. Save as wmv and that could work better.

Or this link may be helpful:

http://www.squidoo.com/mkv-converter-mac

Recently, a poster (ZachP14) suggested this:

“There’s a program called Plex that runs .mkv files awesome on my 2.0 ghz macbook.”

http://www.plexapp.com/

Maybe worth a try?

via Apple – Support – Discussions – Play HD .MKV Files in Front Row ….