First let me say that I have absolutely no experience with a MAC or with Toast. However, it does not matter if you used BitTorrent or not, but if you didn't use BitTorrent you are wasting your time if you don't check the md5 checksum of the ISO that you downloaded against the md5 file on the mirrors.

I saw that someone had added MAC burning info recently to the downloading faq. Of course, Knoppix is not going to boot on a MAC, but you should be able to download, confirm the md5 sum, and burn the iso image properly on a MAC. My guess would be that, if the ISO md5 sum is correct, that you didn't use the correct option to burn the ISO as an image. And, of coure, be sure that you burn the ISO at a slow speed.