When you log in you choose your courses from the available list...

4. eclass List of Department courses in the undergraduate curriculum of DME as an example

ATTENTION in the Category line you have the correct department and study program.
ATTENTION some of the courses at the end of their line have an "open" lock while others are locked. In the open ones, you register directly, while in the locked ones, by clicking on the name of the course, you submit a request and wait for approval from the instructor. Whether a lesson is open or locked is up to the instructor...