I think you're talking about the built-in Comments box for review tasks, which unfortunately doesn't show up on the form itself. You can see it on the Details pain to the right after you open the next user task (or from the Inbox). Click on Action History to see previous review steps and comments. I believe this display will remain selected the next time you open a task.

Alternatively, you can add your own text box to the forms and use that for comments.