MKV isn't supported by anything for editing, you'll need to repackage the audio and video streams in to a supported container like mov.
Your MKV probably contains and h264 stream and an AC3 stream. The H264 stream should be acceptable to video editors but I'm pretty sure AC3 isn't, so that will probably need to be converted to PCM or something. Then you can mux in to mov and open in FCPX.
LOL tutorials?
actually just found this - but it gives a watermark...is there anything for free which does it?