Not easy to work with, but I was able to figure it out.
I added my general location as "Frederick County, Maryland."
Did not work on my iPhone. I had to use my wife's desktop computer. I had to sign in using her Google account (I didn't have a Google account).
I also added White Oak Nursery to the map, along with their address and phone number. To do so, I had to temporarily switch off the Members data layer so I could add an item to the Nursery data layer. It appears that if one user is edting the map, then the system is temporarily muddled for others until they finish their work and switch on both data layers (and then sign out of their Google account).