Q: HOW DO I USE THE AIRPORT EXPRESS

What is your goal for the Express? That is, do you want to use it to provide a wireless network so that one or more wireless network clients can connect to it for sharing and for Internet access? Or, did you get it purely for streaming iTunes and/or for sharing a USB printer on the network?

What is the make & model of the other wireless router that you want to extend with the AirPort Express? Also which exact model of Express do you have? Is your goal to have these two routers connected by wire or by wireless?

Yes, in order to extend the SMC router you would need to connect a single Ethernet cable between the SMC router and the Ethernet port on the AirPort Express or the WAN (circle of dots) port on the latest Express model.

The follow these Apple Support directions on creating a roaming network between these routers.