Q: Trouble using Airport Express to Boost Airport Extreme. Can someone please provide me with some help?

We are using Time Warner's new wideband (Motorola Modem) , I have it hooked to our Airport Extreme and they are in a media closet.  I am runnig Lion on the machines in the house (2 macbooks).  We also have iPhones, Apple TV, iPads etc.  We have 5,800 square feet. Our signal is very weak and is dropped in area of the house.  I would like to boost the signal or "extend our network".  I bought a Airport express to try it out and I can't get it to set up properly. I plugged it into the wall, plugged an Ethernet Cable in and the Airport Utility (I love the new Airport Utility in Lion) shows the Airport Express directly below the Aiport Extreme which is directly below Internet Modem in the network map display. However, the Express keeps flashing amber.  Can someone PLEASE help me.  We have lived in the new house since SEPT, and we have had nothing but trouble.  THANK YOU !!!!!