www.makemkv.com

Same here. And of course 1.6.3 refuses to launch now.

Does your mac have a 9400M, 320M or GT330M graphics card, and is hardware-accelerated decoding enabled in Plex? What settings did you use in HandBrake?

Note that if using HandBrake you want to make sure to have a separate track for the AC3 or DTS core and select that as the source audio track in HandBrake, as it can't extract the core of an HD audio track from an MKV file (it can and will extract the core automatically if the container is MPEG-TS, ...