Use:
0 > boot cd:,ofwboot.xcf netbsd.GENERIC_MD.gz


OF boot command takes three argument.

1. boot device
2. boot file
3. optional argument

1. boot device consist of 
   boot-device:,boot-loader-file-name

   Note that colon(:) delimits device to the left,
   and comma(,) designates that followed name is the boot
   loader filename.

   Boot device name depends what OF recongnize, so that
   it varies machine to machine.  'devalias' command on OF
   may help you to find out the name to use.

2. boot file depends what is written in the boot-device, say,
   CDROM etc.
   You have to know the name of file by other sources.

   Its default is netbsd, netbsd.gz of the booting 
   device. So if the name falls into this category, 
   you don't need to type in.

   If your boot device and the / root partition resides
   in different place, you may say
   root-partition-device:/kernel-name
   to position different root.

   ( kernel-name is usually netbsd and you may say just
   root-partition-device: if it name is netbsd.) 

3. optional argument is
   -s     single user mode boot
   -a     ask boot device
   exit   exit to OF after argument processed
---
Makoto Fujiwara,
http://www.ki.nu/software/NetBSD/iBook2/