In the "My Account" tab, there is a clickable link for viewing gardeners on my watchlist. However, when I click-on the link and go to the list, I can't find the procedure for activating/updating the watchlist, i.e., I can't figure out how to add specific gardener names to the list. Does this tool actually work, or have I missed something?

The way you add vegetable gardens to your watch list is to go to "Shared Gardens".
Fin gardeners in your area and click on the icon to add them to your follow list.