I bought the express and got as far as joining my existing network but now I am not able to get online as my mac can't read the modem or routers.  So I am stuck. Boo. I really thought I could do this. Any suggestions?
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Remember you want to select to extend the network, not "join" it.  If you haven't resolved it yet, try holding down the reset button on the bottom as you plug in the device.  Use a pen to hold the button down first then plug it in to the wall.  Continue to hold the button down until you see the amber light blink at least 5 times then let go.  The light will go solid.  Now wait about 30 seconds until it goes from a solid amber to a blinking amber.  You should now be able to see your airport express in airport utility and can start over the process of extending your current network.