How do I use AUNetSend in GarageBand '09
Assuming you’re using GarageBand ’09 (I think it’s the same in iLife ’11), you can still follow those very same instructions.
AUNetSend is an audio unit that uses Bonjour to send a stream of data in the format that you specify.

You can then modify the properties of the “send”:

The “listening” side should be using AUNetReceive…
I haven’t really tried this since the age of dawn, but assuming both Macs are in the same network, it would be more or less straightforward.