Modifying Lighting Circuit for New 3-Way Lights

Hello, I'm finishing out my basement and need to sort out a 3-way switch for the landing light. Currently at the top of the stairs there's a standard switch. I think the switch is being used to toggle basement lights on/off as well as extend power to a few more lights upstairs near the top of the stairs. Here's a picture of what I think is happening: Attachment 50356 (http://www.doityourself.com/forum/attachments/electrical-ac-dc/50356-modifying-lighting-circuit-new-3-way-lights-current-wiring.jpg) Power comes in and there are two hots on the top screw, one I assume is source and the other goes to 2 lights at top of stairs. Toggling switch doesn't turn these lights on/off. The single hot on other screw goes to lights in the basement.