Although I am not new to video editing, I have never used a greenscreen program. How would I go about replacing the green with a picture or a video. I know to use the ChromaKey FX to remove the green, but I still cannot fathom how to put something in its place.
You're 99.3% of the way there. Place the picture/video you want to use as the replacement on a track below the green-screen video. The ChromaKey FX makes the green areas transparent, and anything on a lower track will show through.