Ok, after reading elsewhere, here is what you need to do in the Key Assignment tab:
 
1. With the mouse, click the keyboard icon for the key you want to remap (in my case the E5) and it shows E5 left of primary/secondary.
2. In the primary (as that is what I want) drop down box, use the mouse to click on Remap.
3. IMPORTANT! You must click the mouse outside of the drop down to close the drop down. IMPORTANT!
4. With the next mouse click, select the keyboard icon for the key you want to be the target of the mapping (in my case L-CTRL) and see the result shown next to the primary drop down. The source key is now shown with an outline to indicate it is remapped.
5. Apply to save all this to the keyboard.
 
Clear as mud :-)