The WRT does not work as repeater. It is a broadband router. It requires a wired internet connection. Wireless clients connect to the router. The router does not connect to another router wireless for an internet connection. It does not repeat. At least with stock firmware.
You may check if you can install a 3rd party firmware on the router. Read the wikipedia article on "WRT54G" for a start. Have a closer look at the dd-wrt wiki which is more extensive. 3rd party firmware may allow you to configure the router as repeater.
Otherwise you have to wire the WRT to your main router. With a wired connection you are able to create a roaming network. That has nothing to do with a repeater. A repeater receives a wireless transmission and repeats it to extend the wireless coverage. A repeater is more or less standalone and does not need wired connections. With a wired connection you can setup an additional base station. This is preferable in most situations as it does not slow down your wireless network like a repeater does.
If it is possible to run a wire between your main router and the WRT you can setup the WRT as access point. See
here for instructions how to connect the routers. It is exactly what badboy suggested.
For a wireless roaming network all access points should have identical wireless settings, i.e. identical SSID (network name, default "linksys" which you should have changed to something unique) and identical wireless encryption (WEP, WPA, WPA2) with the identical keys/passphrase. Only the channels of the access points should be at least 5 channels apart to avoid overlapping. For example choose 1 & 6.
With these settings wireless clients can freely roam between both routers. Or better, they should be able to. Some clients have difficulties with roaming. You should not move those as long as you are connected.
