I assume that you can run a CAT 5e cable from your main router (one of its LAN ports to an LAN port on the WRT120.
Assign the WRT120N a static IP address that is outside of DHCP assignment range of your main router but still in the same network.
Next shut off the DHCP server in the WRT120N, your main router will be assigning IP addresses, so the one in the WRT120N must be disabled. The WRT120 should also be using the same SSID name as the main router.
After setting this up you can connect the WRT120 to your network and have it work as an access point. It is also better if you use two different radio channels on the main router and the access point, you can change those in the wireless configuration of the WRT120. Your AP security should be the same as your main router and use the same passphrase.
So for example, if your main gateway router is 192.168.0.1, allow it to assign addresses in the range of 192.168.0.2 - 192.168.0.75 and make your WRT120 address 192.168.0.76. All subnet mask settings would be 255.255.255.0.