The plugin by SBombauts (Github) for the Plastic SCM source control integration does not currently work on Mac, and on the website for the plugin, it is stated that the plugin is Windows only.
This makes it seem like the plugin supports Mac if you don’t know about his GitHub page, when it in fact doesn’t. Plastic SCM by it self is supported by Mac, which makes it seem like it should work. My suggestion is to remove the plugin from the Mac UE4 editor until it has Mac support, so that it doesn’t lead people to spending a lot of time trying to get it to work, or to invest in a system that doesn’t cover their needs.