You can add comments to any shape in a diagram you have edit rights to.
To add a comment, right-click on any shape and choose "Add Comment."
Alternatively, select a shape, go to the Insert tab and click on Add Comment.
Access Comments by Clicking on the Comment Icon on a Shape
When a user adds a comment, the name they're logged in with and time of the comment will be preserved and added to a comment stream you and your team can access by clicking the blue comment icon in the lower right of the shape.
The Comments Tab
Once comments have been added to your diagram, a Comments tab will appear in the SmartPanel to the left of your drawing area.
The Comments tab will show the latest comment on every shape in the document that has a comment. Each comment in this panel represents a shape in the diagram. Only the latest comment is shown for each shape. You can navigate between shapes by clicking on the comment in the panel. Selecting a comment will open a pop up showing you the entire comment thread over the relevant shape.