Dont think a DM800HD natively supports DivX..doesnt have hardware decoding for this format.
You can apparently stream via VLC player on a PC though, if you install VLC plugin on the Dreambox. I havent tried this myself but it's supposed to work.
I.e: if you have your movies etc on a PC with VLC player, stream to your tv via the dreambox if VLC plugin installed...So the PC is doing all the decoding...