To join a course you will first need to get a 10 character course code from your instructor. Once you have a course code click on the Join Course button on the left hand side menu and enter your course code.
After you submit the course code, Mimir will show you the course name and the name of the instructor associated with the course. If both of these are correct click Confirm Registration to finish joining the course.
On the confirm step you will also be able to pick your section if you instructor has enabled sections on Mimir Classroom.