sounds like bad media. 518 can be a corrupted filesystem, bad bootlist, bad jsf log and several other things. Were this your internal disk I'd tell you to boot to maint shell, run fsck on the rootvg filesystems before mounting then, then run a log form on the rootvg log, reset the bootlist and do other stuff.
But it's your CD and none of that will work on a cd.
Suggest you get a different install CD and try that.