The Netgear Extender, when connecting to it via the Ethernet port, or the Wireless network it creates, should take you to a setup wizard via the web browser. The Setup Wizard will ask you within this setup wizard for your existing Wi-Fi Network's information. The name (SSID), and the password used to connect to the network. It's just like connecting Wi-Fi devices to your network.
A better idea would be to relocate your DSL Router to the first floor if at all possible. That would be better than dealing with network extenders.