>I've tried it on my iMac . Its not working for me. If any one has it working

iMac can boot from internal IDE drive.

Partition map (with 512 byte blocks) on '/dev/rwd0c'
 #:                type name            length   base   ( size )
 1: Apple_partition_map Apple               63 @ 1     
 2:           Apple_HFS*Macintosh HD      2816 @ 64     (  1.4M)
 ...


NetBSD disklabel:
...
8 partitions:
#        size   offset     fstype   [fsize bsize   cpg]
  a:   128000     2880     4.2BSD     1024  8192    16
  b:  ...
  c:  2822400        0     unused        0     0      
  d:     2816       64        HFS        0     0
...

(Partition sizes are only example)

Copy ofwboot.elf to HFS (d) partition, and type from OF console:

0 > boot ide0/disk@0:2,ofwboot.elf

You don't need to installboot.