Okay -- I figured it out (I think). I actually got it working, then couldn't figure out what I did, so had to start from scratch.

On my machine:

Go to "Notation Mode" by left-clicking on the note symbol
Right-click on the stave and change from "notation mode" to "editable mode". You will see little dotted lines running through the stave.
**NOW ** Right-click on the stave where you'd like the section letter to be--the letter will be placed where you right-click.
Select "Section Letters" from the pop-up menu.
Select the letter you'd like.
If you want to remove the letter, repeat the above process; this time right-clicking on the letter and selecting "remove" from the list of available letters.

Thanks Noel and Mac -- I'd post this with a screenshot like Noel did, but I guess you have to be a veteran to have image insert capabilities.

Rhythm guitar is like vanilla extract in cake. You can't taste it when it's there, but you know when it's left out -- Irving Ashby