As MKV files (matroska video) are becoming more popular due to them originating as open source and frequently being used as encoding for H.264. You will find that you might have an issue while either trying to burn them to DVD or playing on portable media devices such as the PS3.
If you have a MKV file you want to burn to DVD goto BHM's excellent tutorial How to convert mkv files to burn as a dvd.
If you just simply want to convert them to another format to play on portable media,just follow the instructions below,
firstly I would like to say a big thanks to reno.99999 for pointing me in the direction of this mkv2vob converter download
I was really surprised at how easy,simple and fast this was to use.
Or if you have multiple MKV file's you want to convert then goto,
Once you have clicked on add file / directory it starts converting automatically
All complete after only a few minutes
I just used the default settings,and all converted fine.Just dragged and droped newly converted file into external hard drive and played on PS3 with out any problems
For more details and info on the advanced settings go Here