it should have tabs to choose wether or not you want to disable/enable subtitles, it should appear after it opens the video, there's a whole bunch of options about what format to convert to, on one of the tabs in the middle somewhere, or it may be the mkv format itself, i remember a long time ago xvid4psp wasn't working well with mkv files and i haven't kept up to date with it, so i don't know it that issue has been fixed or not