Booting a 5000/240 from CD

Hi,

Perhaps a bit stupid question, but I can't seem to figure it out. I try 
to boot my 5000/240 from a freshly created CD (created from the ISO 
image available from the NetBSD site).

I tried something like: "boot 3/rz0" and "boot 3/rz0/netbsd"

Unfortunately it only comes back to me with something like: "?IO: 3/rz0 
       (cmd: rd)"

When I boot the installed 1.6 on the machine, I can read the CD 
properly. file structure looks normal etc.

Any ideas on how to boot the machine from CD?

Many thanks in advance,
Louis