Lightroom is not the product whose extensions/plugins are managed by Extension Manager. In another way of saying, you can't manage extensions/plugins for Lightroom via Extension Manager.
To add SmugMug in your Lightroom, goto Library module, at left Publish Services panel, click the '+' button to open "Lightroom Publishing Manager" dialog. Then open "Plug-in Manager..." dialog, press the "Add" button and navigate to the smugmug plugin you downloaded.
All plugins for Lightroom are managed by this Plug-in Manager inside Lightroom.
Thank you for responding to my question.
However, I still cannot install it into lightroom. When I followed you directions I could not find the file (only folders showed up in the window). I eventually ended up deleting that file from my computer because I had opened it up in the extension manager and could not do anything with it. I cannot figure out how to redownload the smugmug file since all I get when I open my Add On page is either to install or remove. Nothing happens with either option.
Thank you this worked.