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.
Type your comment in the box that appears.
Alternatively, select a shape, and choose Comment from the Insert menu on the Home tab.
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 Control in the Left Bar
Once comments have been added to your diagram, a Comments control will appear in the Left Bar to the left of the SmartPanel and your workspace.
Click the Comments control to see 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.