How to set up my network/bridging using Apple Airport equipment?
Here's what ended up working, via: http://discussions.apple.com/thread.jspa?threadID=2725432&amp;tstart=0

The following are the basic steps to configure a "dynamic" WDS with the 802.11n AirPort Express Base Station (AXn) being extended by the 802.11n AirPort Extreme Base Station (AEBSn). Please compare them to what you attempted to see if anything was missed.

One thing to note is that the AXn is not capable of providing simultaneous dual-band operation like the AEBSn. That said, you will only be able to extend the 2.4 OR the 5 GHz radio of the AXn. Since the lower frequency band travels longer distances, I would suggest extending it.

o If practical, place the base stations in near proximity to each other during the setup phase. Once done, move them to their desired locations.

o Open AirPort Utility and select the AXn.

o Choose Manual Setup from the Base Station menu. Enter the base station password if necessary.

o Click AirPort in the toolbar, and then, click Wireless.

o Choose “Create a wireless network” from the Wireless Mode pop-up menu, and then, select the “Allow this network to be extended” checkbox.

o Next, select the AEBSn, and then, choose Manual Setup from the Base Station menu. Again, enter the base station password if necessary.

o Choose “Extend a wireless network” from the Wireless Mode pop-up menu, and then, choose the network provided by the AXn from the Network Name pop-up menu.

o Enter the base station network and base station password is necessary.

o Click Update to update the base station with new network settings.