You posted under the heading of "Extending the range of your wireless network....".
Is that what you are trying to do?
Will the AirPort Express connect using a wired Ethernet connection.....or connect wirelessly?
What is the make and model number of your "main" Apple router?
What operating system are you using? If you have a Mac, and do not know, click the Apple icon at the upper left corner of the screen, then clidk About This Mac. Post back with the OS X Version number that you see there.