I use C7.2 Pro and am currently rendering over a network. The main machine is a G5, the other two are an Intel iMac and a MDD G4. I have the computers networked through a router. I have left each machine’s settings at the default. If you cannot get it to work any other way, reset the preferences on each machine and see if it works.
Now for a couple tips I learned a long time ago: Make sure the nodes are launched before the main machine’s Carrara is launched. I do not know if this still is required in Carrara 8. I should also point out (just in case) that you need to render through the Batch Queue to take advantage of the network render nodes.