Yes, you connect it to the Apex directly. You can set it up as a wireless bridge or join a WDS network. WDS is really buggy so I don't recommend using it.
Connect it to your current router so you can configure it properly. Use the utility and select "Extend an existing wireless network", then put in your SSID and security information. After that save the config, move the airport over to the apex and plug it in. The light should be green. If the utility still finds the airport after you plug it in by the apex you should be all set. Enable DHCP on your apex and see what IP address you get.