If enabled for your account, you have the ability to add a comment to an activity after it has been successfully completed.
Go to your History Page:
Click the activity title for the activity you'd like to comment on:
At the bottom of the page you will see the option to enter a comment.
Enter your text comments in the box, then click on the "Submit" button. Your comment will appear below the box. Now, people will be able to reply to your comment or add new ones from the Enrollment page of this activity.