Configuring AirPort Express to Extend a Non-Apple Network
Rescan to Find New AirPort Wireless Devices
If your AirPort Express doesn't show up right away click the refresh button or hit command r on your keyboard to perfrom a refresh.
Click Continue to Configure an AirPort Express
Click the continue button in the lower left corner to start configuring the AirPort express.
After clicking the continue button should see a message that says "Reading the AirPort Express configuration...".
Add a New Profile to a Reset AirPort Express
After the AirPort Express has been reset there and a user clicks Continue they should be confronted with a message like the one below.
To setup a new profile, select the bottom radio button that says "Add the deault setting as a new profile", give the new profile a name and then click OK.
Give the AirPort Express a Name and a Password
Give the AirPort Express a name and password (note, this is not the name of the network or a password for the network) and the click continue.
Join Current Network with AirPort Express
Select the bottom option "I want AirPort Express to join my current network" and then click continue.
Extend Current Network with Ethernet Option
Select the "I want to connect AirPort Express to my network using Ethernet to extend my current network or create a second network" and then click Continue.
Select the Wireless Network That is to Be Extended
Select the wireless network that is to be extended. If it's not on the list below then click the "Show More Choices" button.
Once you find the network you're going to extend click on it to highlight it blue and then click "Continue".