Q: Base station switches from green to amber

Is the amber status LED on the AirPort Express flashing or steady? A flashing LED would indicate that some additional configuration steps may be necessary. Which exact model of both the Time Capsule and AirPort Express do you have? Will the Express be used to extend the wireless range of the TC or just use for streaming / sharing a network printer? Finally, will the connection between base stations be via wire or by wireless?

Unfortunately a steady amber status LED would indicate a unit failure. If you haven't already, I would suggest performing a "factory default" reset on the Express to see if this will "recover" it.

I think we need to back up a few steps here for clarification.....then start over again on the setup.

You mentioned a few times that the AirPort Express indicator light was "steady" amber.  Then, you mentioned that it was "flashing".  Which is it?

You indicated in your profile that your iMac was running OS X (10.0.x), but that operating system number is not valid. To find out what operating system your iMac is running.....click on the Apple icon in the upper left corner of the screen, then click About This Mac. Post back with the OS X Version number that you see there.

Thanks for the clarification. Please follow these steps carefully.

Power off the Time Capsule by pulling the power cord from the back of device

Wait a few seconds

Plug the power cord back into the Time Capsule

Restart your Mac

Temporarily, locate the AirPort Express in the same room or close proximity to the Time Capsule

Power up the AirPort Express....if it is not already on

Hold in the reset button on the back of the AirPort Express for 5 to 6 seconds, then release

Allow a full minute for the Express to restart to a slow, blinking amber light status

The AirPort Express will NOT appear in AirPort Utility until it has been set up. You have to run the setup "wizard" on the Express...and then....it will appear in AirPort Utility.  This is likely where you are having the problems.

Click the WiFi icon at the top of the Mac's screen

Look for a listing of New AirPort Base Station

Click directly on AirPort Express just below that

When you click on AirPort Express, that will get the setup "wizard" running. It will take a few seconds to analyze the network and then display a screen that looks similar to this......except that you will see your devices pictured with the name of your network.

Type in a simple, short name that you want to call the AirPort Express

Click Next

The setup wizard will configure everything for you automatically

When you see the message of Setup Complete, click Done

Now move the AirPort Express to a location that is about half way between the Time Capsule and the general area where you need more wireless signal coverage.  The more that you have line-of-sight between the Time Capsule and AirPort Express, the better the network will operate.

What happens if you click on Apple Network a19858?

Do you have an iPhone or iPad handy?

Try the same setup as above, except on the iPhone or iPad home screen.....

Tap on Settings

Tap on WiFi

Look for a listing of Set Up New Base Station

Tap on AirPort Express

The setup is the same as above