If your Macintosh has an Apple-installed DVD-R drive, then you can use iDVD to burn a DVD disk that you create. Don't worry about the format, it'll do it for you through iDVD.
You can also use DVD Studio Pro, if you have it, but if you don't, it's kinda cost prohibitive. I believe it to be around $500.
If you don't have either of those, or you have a 3rd party DVD burner, then you'll need to use Roxio's Toast software. Simply select the "video" tab and make sure that "DVD Video" is selected in the slide-out menu. Then burn those MPEGs away!
If you're talking about copying a DVD, like for backup purposes, you'll have to do a little more work. It's not as easy as popping a DVD in and pressing "copy." Search around on google or yahoo for information on how to do this.