I hope you have already solved the problem.
But by head:
Start the machine till you get the prompt where you can interrupt the boot sequence. Put the bootable cd in the drive.
Then you start a search for the bootable devices by:
in the list you get back is perhaps the first item the cd/dvd
then start the boot from this drive by:
This is for a pa-risc machine; if you have an Itanium the hole is completely different: no experience.