As stated earlier, the default AirPort configuration enables LAN access by default. If you're using DSL or a cable modem, or if you are installing the AirPort on an existing Ethernet network, this is what you want to use. In the Java Configurator, take a look at the Network Connection tab and check the Connect to network through Ethernet port radio button.
From here, you can configure the IP address of the AirPort, either via DHCP, by entering the IP information manually, or by using PPPoE. You'll probably want to use DHCP, unless your ISP requires a manual IP address or PPPoE.