These courses should appear automatically on duo in a box called My Courses.
How does duo know which courses to display? In short, we are fed this information from Student Registry. Before you see your duo courses, you will need to use the Banner Self Service tool to select the modules you are studying this year (unless you are on a programme where they are all compulsory) and your Department needs to approve your choices. Full details of this process are available from the Student Registry website: https://www.dur.ac.uk/student.registry/registration/
Once that happens, your enrolments will appear in the nightly feed of data from Student Registry and you will be enrolled automatically on the corresponding courses on duo. You may also then be enrolled on other related courses, e.g. one for all first year students in your Department or all undergraduates in your Department, based on rules defined by your lecturers and Department.
Note that lecturers prepare and update your courses over the summer, so whilst you may be enrolled on it, you won’t be able to access the course and its content until your lecturers make it available to students.