To watch MKV video on Mac, I recommend trying either MPlayer OS X or VLC Media Player. They are both free. These free multimedia players are excellent and can tackle many of the video formats unknown to QuickTime. They contain a decoder to decode the MKV files. They will also play many of the formats you will find inside the .MKV file.
Although there are some free video converter for Mac, like VisualHub, to convert your MKV on Mac OS X, they are usually hard to use and only support limited formats. For an easy to use MKV video converter, iSkysoft Video Converter for Mac deserves you try. It can do the MKV file conversion like a charm, without any hassle. It can convert MKV files to any popular video formats like MP4, WMV, AVI, MOV, etc.. What makes it excels is that It provides you with the optimized output video settings for iPod, iPhone, Apple TV and many mobile phones. One click to convert MKV to be iDevice compatible.
How to Convert MKV Video Files on Mac (Video Tutorial)