How can I convert MKV files and burn them to DVD?
You could convert the mkv file first with something like Handbrake or iSkysoft's iMedia Converter, then burn it to DVD. Handbrake and iMedia Converter will give you more options for converting than Toast.
Sounds like you need Toast Titanium. Toast will take individual video files and put them into a DVD player compatible format. It doesn't have many options for the DVD's menus, but it's a quick and easy way to burn video files to DVD. It also has many supported formats.