Burning a dvd with 5.1 Surround Sound in DVD Studio Pro

I have a AC3 file for my surround sound. I have played back the audio file on a 5.1 system and it plays back fine using all the channels. Then when I bring in the ac3 file and the mpeg-2 video file into DVD Studio Pro and burn my dvd it automatically down mixes the 5.1 to stereo. Even when I play it back in the DVD Studio Pro it plays back in stereo, even the AC3 file works.

How do I burn a dvd with DVD Studio Pro with 5.1 Surround Sound? Any help would be greatly appreciated. Thanks