A curator is a teacher assigned to a specific student in a course. This is a link in the student list, not a separate role within the course's permissions. They are assigned by the administrator and teachers.
To assign a mentor to a student, open the Courses section, select a course, and then the "Students" tab. For one or more students at once, select the checkboxes and choose "Assign Mentor" (or the same option in the row menu). In the modal, select the course instructor or "No Mentor" and click "Apply." The student will receive a "Mentor" badge—you can change the assignment by clicking on it.
First, add a teacher to the "Teachers" tab of the course - otherwise the selection list will be empty.