Notes can only be added via a person's profile page. To add a note, click on a person's name in the Wall Chart to open the Profile View. Then click over to the section for Notes.
To add the note, select the + sign. Click in the window to add text and make sure to select Save Note when you're finished.
Notes can be viewed by the person who adds them and by anyone who has a role of Organizer, Lead, or Admin. Notes are campaign specific.