You seem to be working through it in a logical manner and have configured it correctly judging from your ability to see the print server's page remotely. But this made me double-take...
But when I try to add the printer as a network printer and put the IP address, the system tells me that it cannot find the printer.
You should be trying to add the printer as a local printer I think. Then for port type choose TCP/IP and so on. Is that what you meant to say or have you tried that (as opposed to adding it as a 'network printer')? I hope it's as simple as that.
Also, the case may be that (since you're trying it from your office) there are port restrictions and settings in place at that end and you may want to consult your local IT guru (if you have one) for permission or further configuration advice.