I can't offer any support, but I do know that I've used a DWL-900AP+ in its WIRELESS CLIENT mode to connect to many ethernet devices. Mostly Linux machines. Why? because I just wanted it to work as if it was connected to the LAN which had the wifi router.
My bet? You need to configure what you have to be a WIRELESS CLIENT to talk to the other Airport.
I have a Strawberry iMac and an Airport base station. Can they be used to access an existing 802.11b WiFi network? If so, how? I have tried by hooking up the Airport to the iMac with a standard ethernet cable (CAT5E) but I can't see any signal on the iMac. The middle light glows a steady green. The outer two lights are off.