Hi
I installed this date picker add-in. Is this the one you tried to use?
Apparently I was already signed into my account, as I was not prompted to sign in. The date picker works as advertised:
In Excel, use File > New From template to display the templates gallery. Click the circle in the upper left corner and see whether or not it says you are logged in. If you are logged in, try logging out, restart your Mac, and then log back in.