No matter, I thought. I opened the widget panel to add it again and was confused to see that there was already a green check mark on the widget, indicating that as far as KDE was concerned, I was already using it.
I looked again but it was no-where to be seen.
A bit of searching around led to an answer.
- Minimize the running applications so you have a clear view of the desktop where the widget should be.
- Right click on the desktop. Select Desktop Settings.
- Under the View > Layout, select "Newspaper Layout". You should see the missing widget(s) at the center of the desktop. (I achieved success using some of the other alternate layouts as well).