After installation you can directly access the plant content through the built-in plants browser. Loading a plant into the scene through the browser will create a procedural geometry object which gives you advanced options about different viewport modes, Level of Detail options, several variations and ages as well as different seasons.

Just start the installer that comes with the App Exchange download. The Maya plugin is installed in the appropriate folder. The content files will by default be in installed to the Library/Application Support/Laubwerk directory. This path is customizable. Since some parts of the Plants Kit are installed into the applications folder, the installer will need admin privileges.