The best results can be achieved using Boris RED as a software filter if you can afford rendering time. Here is a link to a tutorial:
http://www.borisfx.com/tutorials/chroma_key.php
If you are using the latest Media 100 version you should be able to apply RED as a clip effect. In addition, Media 100 has a native chromakey filter under filter panel in the Edit Suite.
Boris Yamnitsky
BorisFX