There are three reasons that you might not see your classes listed in D2L at the beginning of the semester:
1. The instructor has not activated the course yet because it is not ready for student access - Instructors do not have to activate their course sites until the first day of class.
Solution: Contact the instructor of the course
2. You recently enrolled in the course and the enrollment has not yet hit the D2L system - It can take 12 - 24 hours for new enrollments (and drops) to hit the D2L system.
Solution: Wait 12 - 24 hrs.