Thanks to all that have answered. I do, indeed, have the printer. Curiously enough, there are other Macs at home (a MacBook Pro and a couple of iMacs, all of them with WiFi, naturally). There's no problem for all those Macs to "see" the WiFi printer. Unfortunately, doing exactly the same thing on my Mac Pro 1,1 (with no WiFi), the computer won't "see" the printer, unless a USB cable connects both. As soon as I unplug the cable, the printer appears as "disconnected" or something like that. Naturally, for me to add the printer in the first place, it was necessary to use the USB cable, otherwise the printer would be in Limbo and it would appear anywhere. The Mac Pro is connected via Ethernet to the same WiFi router that provides the entire home connectivity (for all the Macs, an HP laptop, a couple of iPads, a couple of iPhones, an iPod and an AppleTV, in addition to three old PPC Macs). As I say, the Mac Pro will simply not "see" the printer. If anyone can think of a reason why, or which specific steps I should take to make it see the WiFi printer, that would be great.
EDIT: My router is a regular WiFi router, not an AirPort Base.