Unzip the downloaded file and double-click the "NintendoSaver2015.dmg" file
Find the image on your desktop and double-click to open it, then double-click the "Nintendo Saver.saver" file inside the image to install
Depending on your security settings, you may see a pop-up window that says "Nintendo Saver.saver" could not be opened because it is from an unidentified developer
Install it anyway by launching System Preferences and clicking the "Security and Privacy" icon
You should see "Nintendo Saver.saver" listed near the bottom; click "Open anyway" to install
Select whether you want to install for the current user or all users to complete installation
Configure the Nintendo Screen Saver through the usual Mac Screen Saver perferences pane:
Launch System Preferences
Click the "Display & Screen Saver" icon
Click the "Screen Saver" tab at the top of the panel to view your screen savers
Select "Nintendo Saver" from the list of available screen savers.
Click the "Screen Saver Options" button to view the Nintendo Screen Saver options window
Select the directory that contains UberNES movies (you can download them directly from the screen saver if you do not have any)
Create a game list by selecting the directory that contains your NES ROMs, which are required for movie/demo mode playback