To use the comment function,
Click on the Annotate icon
Click anywhere on the plan to create a text box
Choose font size, font colour, and box background colour
You can edit the comment box at any point by clicking the arrow and doing double click on the comment box you want to edit.
You can also move it around the page as you need and even rotate it!
For more clarity you can now as well put a cloud or arrow around specific things on the plan and also highlight key parts. See below for an example: