From the slideshow, click the lock icon to open the comment panel. From there, your client can type their comments.
To enter the comments, press 'enter/return' on your keyboard or press "Add Comment" on the screen.
Use the buttons to the right to save comments and continue later, send all comments, or approve the design.
So long as this system is followed, the appropriate person will receive an email update letting them know there are new comments for them.