I'm trying to get a version of Ubuntu running on my old Intel Mac. However, my computers think all of the disc images I've tried so far (downloaded from Ubuntu.com, I've tried 32-bit versions of 12.04, 12.10, and 13.10) are invalid (when I try to open the disc image I get an error "no mountable file system") and when I burn them using Disk Utility (following the instructions here precisely), they make unreadable discs.
There are already a few questions around this issue here, but none of them give any information about what to do (most answers assume the questioner isn't aware of the burn instructions or hasn't followed them; I have).
I've seen some mention of "Mac-specific" images, but I only see 64-bit Mac images and the computer I want to install it on (a 1.83GHz Core Duo from ~2006) is far, far too old for that.
How can I burn a working install DVD?