How do I download films saved on halo 3 from waypoint?
You can’t.
Bungie developed their own custom software to allow film rendering for a cost obviously. When Bungie were passing the keys to 343, they were not required to pass said software over.
If you want to get your Halo 3 clips onto your computer, you’ll have to purchase a capture card or check out www.halorenders.com, it is run by Ov Thy Demise.