by John Provasnik.
It looks like that as long as you 'allow' "File Submissions" then students will automatically be able to make a "Submission Comment" once they have uploaded an assignment. So I don't think you as a teacher would turn this "on" as it will be included when you allow Files Submissions. When you go to grade these assignments, you should see a column for "Submission Comments" -- and make sure it's not collapsed.