The courses can be found on the homepage:
There you will also find the course key for one-time registration in the courses in the course description.
To enroll the courses, log into your account with Moodle and go to the theory course list:
Then click on the desired course title and enter the corresponding course key (see above) once in order to permanently enroll in this course. The course is listed under "My courses" from the next Moodle login - in the lower part of the homepage.