The problem you are having is this, the newer OS 10.6 no longer supports Appletalk. To use this computer you can reload any OS version before , ie 10.5 or 10.4.11 . That will then make your IMac be able to see the older power Mac on the network as long as Appletalk is turned on for all computers on the network . I recently got a Mac pro and had to scrap the 10.6 OS and go back to 10.5 to use this machine on the network. Good luck
People who say it cannot be done should not interrupt those who are doing it,