Summary. The following instructions are for extending an Apple wireless network using the Airport Express. You will need the Airport Utility and a computer with a wireless network card.
Procedure. Follow these steps to extend the range of your network.
1.Reset your Airport Express by holding down the reset button until the status light flashes quickly.
2.Start the Airport Utility and select the "Base Station" listed in the column on the right. Click Continue.
3.Airport Express Name: provide a name. Enter a password: provide a password. These will be replaced when you join your existing network later. Click Continue.
4.Choose, "I have a wireless network and want to add AirPort Express or replace an existing device on my network." Click Continue.
5.Choose, "I want Airport Express to wirelessly join my current network."
6.Choose, "Extend the range of my AirPort Extreme or AirPort Express network." Click Continue.
7.Choose the name of the existing network to join from the drop-down list. This will be the name of the wireless network (not the router name). Click Continue.
8.When prompted, "Please select the Apple wireless device you want to join from the list. It may take a few seconds for your device to appear." The list will show the router name (not the name of the wireless network). Choose the name of your router. Click OK.
When finished, you should get the message: "Congratulations! Your AirPort Express is configured to connect to the Internet, share a USB printer, and play iTunes and music through remote speakers using AirTunes. If you use a cable modem to connect to the Internet, you may need to turn the modem off, wait a few seconds, and then turn it back on. If you want to set up another Apple wireless device, select it from the list."
Kilde:
http://www.resourcesforlife.com/docs/item769