The AirPort Express must be configured using AirPort Utility....the same application that you used to originally configure the AirPort Extreme.
This is a two part process. First, you must make sure that the wireless network that the AirPort Exreme is providing has been configured to "Allow this network to be extended".
To do this, open AirPort Utility, select the AirPort Extreme and click Manual Setup.
Then, click the Wireless tab below the row of icons.
Make sure that a check mark is entered in the box next to "Allow this network to be extended"
Click Update to save any changse that you have made.
When this has been completed, you can configure the AirPort Express to "Extend a wireless network" to provide more wireless coverage as follows:
Locate the AirPort Express approximately 1/2 to 2/3 the distance from the AirPort Extreme to the general area that needs more wireless coverage
Power up the Express and let it run for a full minute
Open AirPort Utility, select the Express, and click Manual Setup
Click the Base Station tab to name the Express and establish a device password
Click the Wireless tab and configure as follows:
Wireless Mode = Extend a wireless network
Wireless Network Name = Same wireless name as your AirPort Extreme wireless network
Check mark next to Allow Wireless Clients
Wireless Security = Same setting as the AirPort Extreme network
Wireless Password = Same wireless password as the AirPort Extreme network
Click Update to save changes and allow 40-45 seconds for the Express to restart and display a green light.
You should be in business
Bob, Thank you. I am able to follow this and everything seemed fine until I clicked on Update. Now, I do not see either the AirPort Extreme or the AirPort Express in the AirPort Utility. I have reset each of the units but now all I get is the yellow light blinking and they still are not showing up in the utility. Please help! Thanks.
Sorry for th problems, this is very strange since my instructions mirrored Apple's instructions for the same procedure as you can see:
Try powering off both the AirPort Exteme and Express. Then power up only the AirPort Extreme to see if it will respond correctly. Then, temporarily move the Express to the same room as the AirPort Extreme and start it up.
If everything is working correctly, move the Express another 10-15 feet further away and test again. You may find that the Express will not work at a certain distance. If that is the case, move the Express closer to the AirPort Exteme as that will be the location that it must use.
Bob, again thanks! I have been able to complete 50% of the problem. The AirPort Extreme Model A1408 is now showing up in the AirPort Utility and is working perfectly. But, I cannot get the AirPort Express Model A1264 to show up and is blinking yellow. I have performed a hard reset but nothing seems to help. Can You? Thanks!
(I have the AirPort Express in the same room as the Extreme.
Make sure that you hold the reset button in on the Express for at least 7-8 seconds and release. Allow 40-45 seconds for the Express to restart to a slow, blinking amber light.
If you have a spare Ethernet cable handy, connect it from your PC to the AirPort Express to help AirPort Utility recognize the Express again. Configure the Express as before. Once the Express has been updated and the settings are stored, you can disconnect the Ethernet cable.
If you do not have an Ethernet cable handy, the Express broadcasts a default wireless network with a name like Apple Network xxxxxxx. Make sure that you are logged onto that network first....no password is required.....then open AirPort Utility and it should "see" the Express so you can reconfigure it again.