It's probably because the iMac is a software base station.
Under the Network System Preference it probably has a real IP address from your ISP while your PowerBook probably has a 192.168.1.xxx type address.
I believe for Rendezvous to work you both have to be in the same subnet (which I think means all the numbers in the IP address are the same until the last .xxx).
You might be able to use something like
Network Beacon to allow what you want to be done, but that's above my head but hopefully not someone else's.