Browse to the folder location of the EarthCache add-in file (*.esriAddInX) that you downloaded on your computer.
Double-click the file to open the Esri ArcGIS Add-In Installation Utility window.
To continue, click Install Add-In. The add-in is copied to the default location and installed. A copy of the file remains in the location where you browsed to it.
If ArcGIS Pro is open, exit and restart the application. The add-in is now available for use.