If you don't yet have any students in your roster:
Log in or go to the Instructor Home. Scroll down to the Courses section and click on the section name that contains the student you'd like to edit. On the next page you'll see your class roster for that section. There's a simple form at the bottom to add a new student.
How to associate an essay with a student:
Go to the Assignment Details page by clicking on your assignment name after you log in. Go to the "grading" tab and find the essay in the list that you'd like to change. In the far right column, click on the edit icon (pencil and paper icon). From there you'll be able to change which student the essay belongs to and override when it was submitted.
Going forward, the system works best when your students submit their own work. The site will then automatically add the students to your roster (so you don't have to manually enter the roster yourself!) and will associate each essay with the submitting student.