(NOTE: the WAP54G device will only work as a wireless repeater for networks using WEP or no encryption, may work with WAP but support is iffy based on hardware version being used on router and repeater)

If router is running DD-WRT or similar firmware go into wireless settings, then to WDS tab, enable "Lazy" WDS or enter MAC address of WAP.

Take a laptop or desktop off the existing network and set it to a static IP in the form of 192.168.1.x with the gateway set as 192.168.1.1

On the WAP login to the config page in a web browser using default static IP (192.168.1.245, pw: admin)

Under basic setup change the IP address settings as necessary to match existing network (use a static IP for the WAP!).

Enter SSID of existing wireless network, select the existing wireless channel, click to enable wireless security and click "Edit security settings".

In the security settings set the Security Mode and WEP type, enter WEP password and save all settings.

Next click on AP Mode tab, select wireless repeater and enter the WIRELESS LAN MAC address of the router. Save all settings

To test your setup go to a wired or wireless computer other than the one you are on and try to connect to the WAP settings page using the IP address that you assigned to it. If you are able to connect than you're all good. As a secondary test you should stand close to the WAP with a laptop and perform the same test as well and make sure you can get online. If these tests fail soft reset both router and WAP and try again.

Don't forget to change the network settings back on the computer you used to config the WAP!