@TsubasaSeraphina
Interesting - did you download X11 from
Apple or did you try to install it from a system disk? Either way, if you've downloaded X11 and it says there's already a newer version, then yes, you've already got X11 installed.
As to VLC, you might want to give the newest version a spin, as their mkv support has improved incredibly - they just need to keep up with the subtitle renderer.
-Andiyar