mac geekery

I have an ancient IBM ThinkPad 380ED that I want to use for an installation of Ubuntu linux. I've downloaded the ISO image from the Ubuntu site, but no matter how I burn it to CD I can't get it to boot on the ThinkPad. So far I have used hdiutil to burn the ISO file itself to a disk, and Finder to burn the contents of said ISO to a CD. Neither works. Inside the ISO image is an "install" folder that contains a copy of SBM (smart boot manager), which went written to a diskette using rawrite does boot the ThinkPad, but produced a cryptic error code when used to subsequently boot from the CD drive. How do I create a working ISO disk using OS X? TIA, Mark