Just drop your composition in a 720 X 480 sized comp. Make sure that the PAR is set correctly. From the image above it looks like you want to use wide screen.
Fit the composition in the frame making sure that the X and X scale values are identical. AE will automatically make corrections for the PAR.
Render the 720 X comp.
Thanks for the quick answer M Gerard.
Actually, I need that stretching effect. So I can't just create a 720x480 composition.
I was doing it before with CS5 and everything was allright. But I had to reinstall the software and now it doesn't work anymore. Maybe it's in the MPG2-DVD settings
Also, what is the PAR?
PAR is Pixel Aspect Ratio.
960 X 540 just happens to be exactly a 16X9 frame ratio.
NTSC DV Widescreen has a Pixel Aspect ratio of 2.12 (this is standard from the Presets)
Contrary to what you may thing or may have heard DVD's are not actually 16X9. There is also over scan... in other words not all rows and columns are visible. You actually can loose as many as 10 lines horizontally and vertically.
If you drop your 960 X 540 comp into a standard NTSC DV Widescreen comp (720 X 480), and scale it to fit horizontally your black lines will disappear, you'll crop off about 2 lines at the top and bottom of your comp, and every pixel will be filled. The scale will be about 90.8333%
You can do this automatically by selecting the nested comp and pressing Ctrl/Cmnd + Alt/Option + H.
This is the proper way to do this. You don't need any stretching effect. If you look at your NTSC DV comp with Pixel Aspect ratio correction turned off (the switch is just to the right of the View pulldown) then it will look distorted, but it really isn't because the 720 comp is rectangular pixels. Turn PAR correction on and you'll see exactly what you want to see.
BTW, the reason you can't render directly from AE is that NTSC DV MPEG compression for DVD is set by the codec and can't be altered.
One more thing, if you want to look at this output on the WEB then you MUST compress from a comp that's square pixels.
I'd suggest that you do your encoding for DVD with Encore or Adobe Media Encoder. AE cannot do multi pass compression and that is critical for quality encoding.