I thought that this question had been answered in your other post today.
The how to use the Difference Matte is described in my blog post whose link I have given you in that other thread.
Some try to use VIdeomerge in place of the Mac missing Chroma Key, Blue Screen Key, and Green Screen Key effects.
Is this thread an ovesight duplicate? If we missed targeting your question previously, please give more details on what you seek.