Unfortunately the Mini Toolbar in Word 2010 and 2013 can't be customized to add an "Add Comments" button.
However, here are a few other ways you can accomplish this. After highlighting your text using the keyboard or a mouse, do one of the following:
- Keyboard: Press CTRL-ALT-M. A new comment will be created and your cursor will be positioned ready for you to type your comment.
- Mouse: Right click and select New comment which is the last option at the bottom of the context menu (available in Word 2013 or later).
- Keyboard or Mouse: Press ALT-R (or click the Review tab on the ribbon) and press C (or click new Comment).
You can also add an Add Comment icon into your Quick Access toolbar in the top left of Word.