I used to be able to do it in Compressor. It took ages for it to accept a mkv file that I had dragged into it, but after a VERY long while the spinning beach ball would stop and it would appear in the window so I could apply whatever DVD settings I wanted.
However that seems to have stopped working. But if you have an older version of Compressor or haven't upgraded your system in a while you might try it.
Otherwise, if you have the late lamented Visualhub (or are able to find a working version) you can convert the mkv directly to a DVD.
Or you can use Handbrake to convert the mkv using the best quality settings possible and then use Compressor (if you have it) to make a DVD
Those are all things I've done so I know they work.
I haven't done this but it seems to work - if you have Toast, drag the mkv into the Video tab with DVD-Video chosen.