Ack! Typical! Minutes after I post this request for help . . . . I figure it out on my own!
Earlier, I was trying to add a new yearly subscription to D&DI
while signed in to my account. Apparently, all I had to do was
sign out, and the "add subscription" button magically appeared!
While I'm happy I now have a yearly subscription, this experience has not been a pleasant one. I really hope that
WotC straightens out their online billing system, as I imagine it is turning away potential customers.
And finish the various D&DI tools of course . . . .