Most of the Sun machines these days have 2 levels of connecting and three different physical port to do so.
One level is connects you to the SC controller where you can manage the hardware. There are two ways to connect to this level. One is a serial connect and one is a network connect. When you first get the box you can only connect through the serial.
Once connected you can run "setupsc" to configure the network connection. To connect to the serial port you should have gotten a serial adapter with the box. Use that with a cat5 cable to connect the serial port on a working box to the RJ45 serial port on the new server (looks like a regular network port, but it is serial) using something like hyperterminal if you are on Windows
Once the network port is configured, you can connect through either the serial or network connections to connect to the system controller (SC) (not the OS). You can 'jump up' from the SC to the OS level with the "console" command.
Once you have installed an OS and configured the eth interface, you can come in through that interface also.
At this point you will have 3 RJ45 ports avail:
Serial (sc)
eth (sc)
eth (OS)
Good luck