If you have proper mkv movie files in your shared directory, but don't see those files when you are sharing to your media player using the Wireless Media Stick™; there are 2 possible reasons for this -
1) Your media device does not support mkv files (to check this, copy one of your mkv files to a regular usb thumbstick, and plug it into your media player, and see if your player can read the same file off of a regular USB stick. If it can't, it is because your player doesn't support mkv files. This is true for any file type.
2) The mkv file is larger than 4 GB. The Wireless Media Stick™ currently only supports file sizes of 4GB or less.