Wireless routers cannot be used as wireless clients. Some wireless access points can but it depends on the model. In your case you are lucky that you have a WRT54GS as these can be flashed with third party firmware that does allow client operation.
Examples of this firmware are
Sveasoft,
OpenWRT and
DD-WRT. I'm not sure which ones specifically support operation in client mode.
Bear in mind if you set it up as a wireless client you can only connect one computer at a time if you want the wireless link to work properly. This is due to limitations in the 802.11 standards.