You must be enrolled in a topic to see class options in the My Timetable tab of the Student System.
Make sure that you have selected the correct semester period in the 'Class Registrations for...' drop down box at the top of the My Timetable screen to display class options for the relevant enrolled topic.
Enrolment and class registration opens from November for study in the following year, with staggered opening dates depending on the subject area. Check out the Enrolment and class registration schedule which includes the earliest dates that you can enrol in topics and register for classes.