All of the documentation I can find on Apple Support states that I should have an option I can check to allow the wireless network to be extended while in the wireles setup section for the Airport Extreme in Airport Utility; but, that check option isn't there.
You are likely looking at older documentation, which would not apply to AirPort Utility in Lion or Mountain Lion.
No doubt, you are using Lion or Mountain Lion on your Mac, correct?
If yes, post back for more instructions.
Close AirPort Utility if it is open.
Temporarily, move the Express to the same general area as your AirPort Extreme and power it up and wait a minute.
Hold in the reset button on the Express for 9-10 seconds and then release. Give the Express a full minute to restart to a slow, blinking amber light.
Click the AirPort icon at the top of the Mac's screen and wait a few seconds for a listing of New AirPort Base Station to appear
Click on AirPort Express just below that and AirPort Utility will automatically open
Wait a minute while AirPort Utility analyzes the network and announces that the Express will be configured to "Extend" the AirPort Extreme network.
Enter a device name for the AirPort Express and click Next
Wait another minute to allow the configuration to occur.
When you see the Setup Complete message, click Done.
Now move the Express to a location that is approximately half the distance from the AirPort Extreme and the general area that needs more wireless coverage
Power up the Express and you are all set. Trash those old instructions.
I am having a heck of a time getting my AExpress (bought today, current rev) to extend my Airport Extreme v2. I thought it best to comment on this thread rather than open a new one.
I have done all of the set up procedures described in the manual and in this post multiple times and I seem to always end up with a flashing yellow on the AExpress.
I have set the AExtreme so that it may be extended and I have reset the AExpress numerous times.
My network is an unsecured. Does that matter?
My problem (since resolved) was that the AE is somewhat sensitive as far as its ethernet/network wired port is concerned. The connection I was using had previously run a networked printer on the same network without issue. I connected the AE to the same Cat5 cable and the AE would not obtain an IP. As soon as i tried another in the house it worked fine. Wish I would have tried that first.