did you change the LAN IP of the WRT54GL to match the IP scheme of your main router? I like to use the main routers LAN IP but ending in .253 since some router's DHCP range starts at .2
Open the Setup -> Basic Setup tab
Connection Type will be: Disabled
Set STP for Disabled (Enabled sometimes can cause connection problems)
IP Address : 192.168.1.2 (Assuming Primary Router IP is 192.168.1.1)
Mask : 255.255.255.0
Gateway: 192.168.1.1 (again assuming Primary Router IP is 192.168.1.1)
DHCP Server: Disable
Local DNS: 192.168.1.1 (if IP of Primary Router is 192.168.1.1)
Assign WAN Port to Switch : Optionally enable this to use the WAN port as another LAN port.
Save
http://www.dd-wrt.com/wiki/index.php/Repeater_Bridge