As far as the nodes not being scene.  Here are a couple of general suggestions from a network admin.  Note that* * I am not going to be too specific about setting up the Macs since I haven't touched one in a few years, and honestly do not remember much of there setup system.

1.  Set each machine up with a static (Hard, fixed etc.) IP address.  Your router's lan side should be using something like 192.168.0.1 .  You will first need to get the hardware address for the internet adapters on each machine.  In the router, on most routers, you can assign a specific IP address perminently to a certain hardware address (sometimes called a MAC address(no relation to the type of computer)).  You merely set up each machiens address in the router to assign to one IP from the same block as the router.  any address between 192.168.0.2 and 192.168.0.254 can be used.  Also make sure that each MAC has the appropriate address set up in the TCP/IP properties in advanced mode.  You also want the Router's address set up as the gateway.  The router will pass DNS around so you don't really need to put in a static reference for a DNS server unless you want to ask your ISP for the addresses for their DNS servers. 

Another issue may be your firewall.  I would set up an exception for C5 and the render node software.  You may also need to open up the appropriate port in the router.

 

I hope some of this helps.  Remember when in doubt call applecare. :)

 

Rich