Why did you use the old mac os version? It will be better to use with 10.15.4 and above because it has better Metal 2 api (from performance point of view) & better apple-amd drivers.
Try to delete the previouse version manually and move "rprblender" folder from zip in the same manual way. Active it from blender pref after that manipulation.
The folder path is /Library/Application Support/Blender/2.8x/scripts/addons/
I don't have the problems like you. Smth wrong with your files.
