Instructions
Open the PS3 Media Server program on your computer. Use the icon in either the "All Programs" menu or on your desktop (whichever is applicable).
Click the "Transcoding Settings" button at the top of the screen.
Select the "Force transcode for following extensions" option. Check the box next to the "MKV" file extension listing.
Click "Save." PS3 Media Server will transcode all MKV files into an appropriate format before streaming them to the PlayStation 3.