Opening .MKV files

Hi, I recently downloaded the trial version of MP3tag on my Mac to add some thumbnails to .mkv files Ive ripped. However when I find a .mkv file to open, it's greyed out and doesn't allow me to open it. No problem with .mp3. But I'm not able to open .mkv files to make edit. I thought this software was able to make edits to .mkv files. Am I doing something wrong?

see here for supported file types: