After living with this for a week or so, I discovered the issue and how to correct. Note that I'm using Excel 2007, so your version may be different.
- Click the Office Button (upper-left round Office button).
- On the bottom of the menu, click Excel Options.
- This will open the Excel Options dialog box.
- Select Advanced from the left-menu.
- Check the box, Allow editing directly in cells.
- Click the OK button at the bottom of the dialog box.