from the OK prompt to see what the device path into the cdrom drive you want to use is.
You can then use that info to load the boot kernel from partition f of the CD
For example, a SCSI cdrom at target 4 on the motherboard SCSI controller of an Ultra/1 would display as /sbus@1f,0/espdma@e,8400000/esp@e,8800000/sd@4,0
Therefor to boot from this at the command prompt, use the following
ok boot /sbus@1f,0/espdma@e,8400000/esp@e,8800000/sd@4,0:f
On an Ultra/10 with an internal IDE boot disk and an external cdrom at SCSI target 5, attached via the optional SCSI interface,
ok boot /pci@1f,0/pci@1/pci@4/SUNW,isptwo@4/sd@5,0:f
The 4's are from the PCI card slot location of the scsi interface, so