We have lift-off.
As to the Calendar, on your iPad open the Calendar app then touch Calendar at the bottom of the screen and compare the calendar name you see there with what you see on the iMac in the Calendar app when you click on the Calendars button at the top left of the window. Those names should exactly match.
The names were slightly different (one had national holidays, the other didn't). After that was corrected I went into iCloud, turned Calendars off, then back on, and things started working. I was able to add an event on the iPad and watch the change on the iMac in real time.
Thanks Ira, thanks joemike.
