tman,
We use the latest and greatest drivers from hp (I found this to be critical)
and then set the plotter up as a regular system printer through the windows printer setup. Once we are sure we are communicating with the plotter via the pc/network, then we set up the plotter through then AutoCAD plotter manager. We had issues at first, but these were due to having incorrect drivers, the 1055CM tolerates only drivers specifically written for it, again, they are available on HP's website.
For grins, you might try connecting the plotter to the back of your pc via a Centronix cable and see if you are able to plot this way, if not, make sure you aren't plotting to a file, or non-existent spooler. If you are successful plotting with the plotter connected directly to your PC, you now know you have a network issue.
Another tool you might find useful from HP is their HP Install Network Printer Wizard (also a free download from HP).
The reason I suggest this, is the printer wizard can handle all those tricky port configurations for you - all you have to do is specify an IP Address, or printer name (the wizard can also scan your network for available printers) and then, if you have downloaded the latest drivers, the wizard will ask you to either select from a list of printers your computer knows about, or supply a location of where your setup drivers are located, and it can perform the printer install for you. I found this eliminated a lot of errors when trying to set up a printer/printer port manually.
Hope the helps
Todd