Here's what ended up working, via: http://discussions.apple.com/thread.jspa?threadID=2725432&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.
From the Apple Support Airport Express FAQ:
Question: Since AirPort Express only has one Ethernet port, does it act as a LAN port, WAN port, or both?
Answer: Depending on how you have it set up, the Ethernet port can function as a LAN port (defined as either simple bridging or sharing
your Internet connection via network address translation) or a WAN
port (connecting to your broadband Internet service provider or
upstream router). However, it never acts as both simultaneously.
By default the AirPort Express Ethernet port acts as a WAN port.
When it is set up as a secondary extended Wi-Fi base station, it can act as a LAN port. Assuming that your Internet service provider only
gives you one IP address (the most common scenario for homes), this
means that AirPort Express can only accommodate wired clients when
acting as an extender or bridge. Therefore, if you need to support
wired clients and only want to get one base station (or router, as
it's the routing capacity of AirPort Express that is relevant here),
you should get the AirPort Extreme Base Station or Time Capsule instead.
According to an answer to a question on an Apple Store page:
Yes, certainly. Connect Ethernet cable from the Express to your
non-wireless computer which meets the above listed "System
Requirements" and use AirPort Utility to direct the Express to "join"
your wireless network from the list which will show on the popup
presented to you. The Express will then function as a wireless adapter
(receiving signals) for your computer, but in that configuration it
will not extend the wireless network to other devices.
You can accomplish this task via the Airport Utility. In one Apple discussion forum discussion, a user (Bob Timmons) notes that:
new AirPort Express 802.11n can be configured to "extend a wireless
network". That is a much simpler setup with much higher performance
than WDS can offer.
You'll need to use Apple's Airport Utility to set up your Airport Express. Two approaches that you can take are WDS or Extended wireless network, as noted in this Apple Support KB.
Best Answer
This doesn't answer your title, but your question I suppose. Unless you just have the AirPort lying around without any use, a cheaper solution would be to get a USB Wireless Adapter. You can get those for almost a tenth of the price of an Airport.