Not sure about .flv but I know it will work with .mkv .avi .mp4 AVStoDVD - Video software and downloads - VideoHelp.com
I think it's better to output a VIDEO_TS folder then burn with Imgburn. Also free. There's a guide on the Imgburn forum how to burn DVD9 with setting the layer break etc..
I use a custom DVD size of 8032 MB rather than full DVD9 and set chapters every 5 minutes. That way the automatic "set optimal layer break" in Imgburn can set the layer break for you like 99% of the time.
If your DVD player can play .avi/.divx then an alternative may be: http://www.videohelp.com/tools/Quick_AVI_Creator
I've just started using this but it seems to give good results. In fact I edited the xvid.presets file to add an xvid "one pass mode"
It uses AvsP for AviSynth scripts. It seems daunting that it pops up an editor with this script, but unless you are downsizing from HD to SD you usually do not need to edit the script. Just go into the Tools folder and click Encode Video. Plus it can encode with x264 if you have a set top box. Great for HD with manageable file sizes.
I see the .divx company itself is switching to .mkv for it's divx players in the new generation called "Divx Plus" so it looks like .mkv is the new way to go.