I use the Chroma-Key a lot, so I can help you out there. Which version of Movie Edit Pro do you have?

The basic understanding for using this feature is setting up the track correctly. Remember that you need to use the Time-Line Mode of the Editing set. Here you will see each track. Note that the lower number track is the background to any higher number track. This is useful to know for Chroma-Key, Picture-In-Picture and Text overlays.

Once you have your background image or video in the arranger, you then load in the foreground, this is the one that will have the Chroma-Key color. You then select that object as it is in the track (it will highlight orange) then depending on the version of Movie Edit Pro you have (I am assuming 16) then you go to the Effects Tab, then to Video Effects, and then choose Chroma-Key. You then select the color that represents what is in the foreground image or video.

You then can manipulate the fade range and threshold of the effect.

I hope this was useful