AirPort Express: Difficulty joining third-party network that is using WEP
Symptoms
Joining the network of some third-party wireless access points requires a little special treatment when the network is protected by Wired Equivalent Privacy (WEP), an older wireless security protocol.
Products Affected
AirPort Express Base Station
Resolution
First be sure that the third-party access point is not restricting clients based on MAC addresses (sometimes called an access control list), and be sure that the base station has the latest version of firmware that's available from the manufacturer's website.
Next, beware that WEP passwords can take either a plain language or hexadecimal form, as described in joining an encrypted wireless network. You might be able to avoid password issues entirely by changing the password of the WEP-protected network to a better format.
Using that information, you should try connecting an AirPort-enabled computer to the WEP network to verify what will be required. If you have to enter a hexadecimal value to get the computer on the network, the same will be true for AirPort Express.
Confirm that you are running Mac OS X 10.4 or later or you have the AirPort 4.0.1 update installed. Open AirPort Utility, and go through the steps to set up AirPort Express to join your existing network. When you get to the page where you select the wireless network the AirPort Express is to join, you will see two password entry fields. If a hexadecimal password was required to get your AirPort-enabled computer on the network, use the same hex code here, but add a "$" in front of it.
So if your hex value is "6170706c65", you would enter "$6170706c65".
Finish the steps in the Assistant. Once the AirPort Express has had a minute or so to restart, it should join the network and once it finishes rebooting (allow up to a minute) it should join the existing wireless network, as indicated by its status light.