SUCCESS! I just got off the phone with one of the head printer engineers at Hewlett Packard. The basic problem was that BOTH the Belkin router and the Airport Express were supplying IP addresses via DHCP and they were fighting with each other. Using Airport Admin Utitlity, I turned off "Distribute IP Adresses" in the Network Tab. Basically, this turns the Airport Express into a repeater for the Ethernet router.
Now the MacBook is getting it's IP address from the Ethernet router, as is the printer, and everything is fine. All computers, wired and wireless, can see each other in the network browser and print to all the printers.