As one who has a tendency to miss some easy items myself (as seen by my request for help earlier today :-)), I thought I'd ask...
Are you trying to click the text box itself and are not getting the chance to type in it? If so, have you tried typing directly in the Comments List?
- In the Comments List, click the entry for the text box. (This should select the box.)
- In the entry in the Comments List, click just below the comment header info ("Page n mm/dd/yyyy hh:mm:ss PM"). If you're lucky, the text field will appear (white box) and you can type your note in there.
- When finished, click outside of that entry. The entry is saved and the text appears in the text box.
This is all dependent on the item actually being a text box, not a rectangle :-)
(If it's a text box, the icon in the top corner of the entry in the Comments List will have a 'T' inside of it.)