Here's how to do it:
Select the requirement you want to add a note in. You will know you selected the correct requirement when the information for it is displayed above the "User Notes" section on the right side of the page.
Next, go to the bottom right side of the page to the box that is labeled, "End User Notes (Optional)." You can type your note here.
Once you complete writing your note, select the orange "Send" option, located to the right of where you typed your note. Your note will then be displayed in the "User Notes" box.