welp, an ISO is simply an image of a CD. By burning from an ISO you're making an exact copy of the original CD. So, if the cd you're burning wasn't originally bootable, you can't make that one bootable. If all that was on the CD was a bunch of RPMs, all you'll see is a bunch of RPMs.
What exactly are you trying to burn? A mandrake install cd? Check your BIOS settings to make sure it's set to boot to cdrom first. Maybe the CD really is bootable but your computer isn't trying