Ah, I'm very new to this or I would have immediately noticed that there is no 'evaluate' link.
The problem then is that there is no 'evaluate' link following the cell. I did try shift-enter, thank you for that, but that also had no effect.
The cell is outlined in blue. In addition to the non-appearance of the 'evaluate' link, none of the buttons, such as the 'Save' buttons, work. This points to a javascript issue. However, javascript works fine in other web pages.
I tried deleting the dot-sage file from my home directory and re-starting sage, but still had the same problem. I then removed all the sage files and re-installed. That also did not fix it. Right now I'm out of ideas.