I have a older blue iMac that is running Debian linux.(An old version of about 3 years ago without update. Doesn't run well after loading) At the choice of loading linux from from hd or cd, I choose CD, then the program drops down to CD..., and then nothing. The Ubuntu cd is from this site. I tried desktop and also server. Nothing. How do you mend a broken prompt?
Does your iMac look like this: http://en.wikipedia.org/wiki/IMac_G3 ?
Those were using PowerPC processors, you'll need PowerPC version of Ubuntu. https://wiki.ubuntu.com/PowerPCDownloads