When you say you've tried to enable it as an add-in, what happens? Are you getting an error? Do you even see Esri Maps for Office listed as a choice? Can you check the box for it under COM Add-Ins and click OK or is that not even an option for you? I'm not sure why the behavior in Excel would differ from that of PowerPoint if both applications are from the same suite of products. As an extreme means of troubleshooting, you might try uninstalling Esri Maps for Office entirely, then rebooting, then re-installing it being sure to use the correct URL for AGO or Portal (whichever product you're connecting to via EMO).