I have just installed a third-party extension to ArcGIS Desktop 10.3 (Land Change Modeler for ArcGIS 2.0, essentially IDRISI ported to ArcGIS, as far as I know). There were no error messages during installation, and nothing to indicate that the extension was not installed properly. Yet, when I opened ArcMap and tried to activate the extension, it was not one of the choices under Customize --> Extensions.
Obviously, ArcGIS Desktop 10.3 is blissfully unaware that LCM is installed on my machine. Is there any way to manually register the extension with ArcGIS? Both the LCM an the ArcGIS files are installed in folders in the Program Files (x86) directory. Clark Labs, the maker of the extension, told me that this (and not somewhere inside the ArcGIS folder, for example) is the right location for them.