Okay, I'm officially a dumbass.
On the "Setup Map" page, highlight the little "i in a circle" icon, which allows you to check or uncheck the various topo sheets that are loaded, then press the "Menu" button, which gives a dropdown menu that permits you to restore defaults, show all maps, show none, show the basemap. show California Topo, etc.
The basemap was there all along - I just didn't know that there was a dropdown menu that could be accessed from this particular page if you pushed menu.
Color me relieved. And a dumbass...