Okay, so in searching I found that I could download the SmugMug plugin directly from Smugmug here:
The one I downloaded from Adobe was zipped, which I guess requires use of Extension Manager to unzip the file. I'm not a subscriber to Adobe's creative cloud so EM would not run. I purchased LR from Amazon as a stand alone product. Seems to be the case that I should be able to access plugins from the Adobe site without having to "rent" EM. I dislike Adobe's model to rent software.
You don't use Extension Manager to add plugins to Lightroom - you simply open the package supplied by the vendor (using WinZIP, etc.) - you will find a folder inside with a name that ends in .lrplugin. That entire folder is "the plugin", so it must be moved to Lightroom's presets folder. The location of that can be found from the preferences dialog on the "Presets" pane. It will open the parent folder - browse inside it for the "Modules" subfolder into which you copy your .lrplugin package. Restart Lightroom and it'll load.
Thanks Dave for the information.