Click on the Notes icon above (try it now) to enter Notes.
 
Notes are limited to 255 characters. You probably should keep your notes somewhat brief, as they may run off the page when printed. However, if you export your TimeTracker document and then import it into a database program, such as Excel or FileMaker, then there should be no problem.
You can use the usual Macintosh edit commands when entering notes (cut, copy, paste, and clear).