If you're viewing a storyboard in its shareable view, adding comments is done from within the comments panel bar. As you cycle through your frames with the comments panel bar open, you'll notice the new comment input box at the top of the panel bar.
Adding a comment:
Hover over a frame and click on the Comment button.
This will open the comments panel bar where you can enter and save your comment. If you don't have a Boords account, you'll be asked to enter you name here too.
The comment box will automatically update to match the frame you're currently viewing.