Green Screen Keyer issues

You have to be more specific: how exactly is it not working? What happens?

As LocaAlicia said, some screenshots could help.

Please include the timeline, the viewer and the Inspector showing the clip and the Keyer applied

Edit: My bad, I was making a big mistake. It works just fine.

Be sure your talent is not too close to the screen (min 10 feet if possible), so the green light doesn't spill onto them. That makes keying much harder. A backlight on the talent helps a great deal, also. And make sure your green screen is very even in color, evenly lit. Get rid of wrinkles fast and easy by clamping top and sides (weight bottom if you can) to stretch the screen material. Then mist it all down really well with a water spray bottle, when it dries, wrinkles will be gone.

Does your background clip (below the green screen clip) actually fill the entire frame?

Go to the first frame of the clip, in the Inspector, in the Green Screen Keyer section, click the Sample Color button, then draw a small square on an area of the green screen, away from the talent. Does that not work?

I can't understand from the text alone, so please show me a screenshot.