you will have to configure the WRT54G2 first be for connecting to the rest of the network.
log into the router, disable DHCP, assign 192.168.1.254 as the LAN IP, then save.
nest setup you wireless security, I like to use WPA2.
when done connect the WRT54G2 via network cable by connecting one of the four LAN ports (NOT the WAN/Internet port) of the WRT54G2 to the one of the LAN ports of your main router.