Close your Cinema 4D application. Double-click on the RealFlow | Cinema 4D package. Follow the dialogue, choose the destination drive and activate the Cinema version on which you want the plugin to be installed.
After the installation, start Cinema 4D. The plugin appears under “RealFlow” in Cinema 4D's main menu.
If you have not licensed the plugin yet, it will launch the License Activator so you can easily do so. It is explained here: macOS Licensing.