AirPort Express: Difficulty joining third-party network that is using WEP
Joining the network of some third-party Wi-Fi routers may require additional steps if the network is protected by WEP - Wired Equivalent Privacy (an older wireless security protocol).
First be sure that the third-party Wi-Fi router is not restricting clients based on MAC addresses (sometimes called an access control list), and be sure that it has the latest version of firmware that's available from the manufacturer's website.
Next, be aware that WEP passwords can be in plaintext (WEP 40/128-bit ASCII) or hexadecimal (WEP 40/128-bit hex ) format. You might be able to avoid password issues by changing the password of the WEP-protected network to a more recent and secure protocol such as WPA2. Learn more about password formats.
If your Mac requires a hexadecimal to connect to the Wi-Fi network, the same will be true for AirPort Express.
Configure the AirPort Express to join the Wi-Fi network:
Open AirPort Utility and proceed through the steps to set up the AirPort Express.
When you select the existing Wi-Fi network, you should see two password entry fields. If a hexadecimal password was required to get your computer on the Wi-Fi network, use the same hex code, but add a "$" in front of it. Example: For hex value "6170706c65", enter "$6170706c65".
Complete the AirPort Utility setup, and allow the AirPort Express to restart.
When the AirPort Express has restarted, it should join the network (allow up to a minute). If the AirPort Express has been configured correctly the status light will turn solid green. If it continues to flash amber, use AirPort Utility to determine why.