Start up the Blender Publisher. In the File menu (top bar, left) you can find a new option: "Install Key". This invokes the typical Blender file-selecting window, browse to the location where you saved the Key, select the Key file with LeftMouse, and press the button "LOAD". This action will make Blender copying the Key to its default help file directory.
When the install went right, you get a new splash-screen, a new 'P' icon in the top header, and extra options related to the Publishers functionality.