I am using a JetDirect EX Plus. It is on a network with a router. The router is set up as DHCP so it dynamically assigns IP addresses to the various machines on the network.
The print server keeps the same IP address as long as it has power. If there is a power failure I simply start the print server before anything else and it is given the first IP address available.
In print center, select add printer. Choose IP printing. Enter the IP address. Select HP printer from the pull down menu and choose 455CA using gimp...
Before I do this I sometimes check the IP address of the print server by pinging it. There is a network utility in the utilities folder of OSX. I watch the lights on the print server and ping it. If I have the right address, the lights will flash and the utility will return a positive result.
There may be a better way of doing this, but that is how I do it. BTW, when I bought the plotter, HP told me to throw away all of the software that came with the print server as it is Windows only.