I've found something that seems to ACTUALLY WORK, and it's super easy!!!
After failing to get media player to communicate with my 360 reliably... After trying half a dozen MKV converters that either completely fail or generate a choppy unwatchable video...
I heard about a program called Vuze. All I had to do was install it and turn on my 360. My Xbox 360 shows under devices and I just drop in a MKV video and it converts it. Only downside is it takes about 3 hours to convert a 90 minute movie but unlike EVERYTHING else I've tried, it actually works!
Did I mention it ACTUALLY WORKS!