There is no Chroma Key, Green Screen Key, or Blue Screen Key in Premiere Elements Mac. Those are Windows Only features for Premiere Elements.
Many suggest Videomerge as the workaround. I do not favor Videomerge.
Please review and consider my suggestion of the Difference Matte as the workaround. It has worked for many a Mac user in a situation such as the one you are in.
Please review and consider. Any questions on this or need clarification, please do not hesitate to ask.