There is no knoppix for powerpc. You should read in the utubtu forums for basic information.
What you have a is a live cd, where you can boot to it and use basic functions, and also install it. But again, it will only work on an x86 processor.
Good luck with linux, we have it on 2 computers in my home and we love it. My brother actually has it as his principle operating system on his laptop, and he's 2nd year biology!
You will find everyone in the ubuntu forums (much like MacRumors forums) are very helpful and understanding.