A couple of options come to mind:
1. Select the text, click "Insert", "Comments", type in your comment
If you aren't really "commenting' on a document, but just wanting to
highlight certain things,
2. Open the Drawing Toolbar, click the oval tool and draw a circle or oval
around the text, click the textbox tool and draw a textbox to the side and
type in your comments. You could even draw a line with an arrow pointing to
it if desired.
