1. Add a camera to Global Sources.
2. Add that camera (from the new 'global sources' option in the right-click menu on the Sources list) to a scene.
3. Preview the scene. You should see your camera's video.
4. Right-click on the global source in the list, select 'properties'.
5. Check the 'Use Chroma Key' box. Click the 'Select' button. Move your mouse to your preview window, and use the now-eyedropper to select an example color (you want to hit the middle of your color-range, so not a shadow and not a highlight).
6. Fiddle with the Similarity values, and the Blend until you get a good, clean chromakey. Add spill reduction only if part of the edges of you are being chroma'd out (spill).
I use a Similarity of 80 and a Blend of 60 on my own screen; it's really a question of how good a greenscreen you have, and if you've lit it properly.