How to install actions in Photoshop
The easiest way to install actions in Photoshop is to drag and drop the actions file on the opened photoshop window. This should work both on Mac and on Windows.
Caution! Photoshop stores actions in the default photoshop preferences file. This file may get corrupt and must be deleted or is replaced automatically by photoshop. Keep your actions file somewhere for safety.
However, photoshop actions can be installed 'by hand': Put them somewhere on the file system, then in photoshop open actions palette and click the tiny triangle in the upper right corner. From the appearing menu choose 'Load Actions', then choose the file from the file open dialog.
On windows there are actions in some subfolders of the photoshop installation folder depending on photoshop version, preferable in 'Required' and 'Presets\Photoshop Actions'.