The .iso files are disk images. Under Toast (for example) you must select Disk Image from the 'Other' menu to properly burn an iso. Then you simply select the .iso file from the hdd, and click Record.
In fact, I just burned and booted a Disk 1 valhalla CD for a buddy: he could not seem to burn a bootable CD. Whatever software you are using, read up on the way to properly burn disk images.