Once a colleague adds you to her course, you should get an invitation in your inbox to join that course. Once you accept, it will appear on your dashboard. Depending on which role you were assigned (colleague, co-teacher, observer...) your access to the course and how much you can edit/interact will vary.
In case you cannot see the course, you may want to check "courses" in the sidebar. It is just underneath the dashboard logo. If you then scroll down to "all courses" you can also see courses which you have removed from the dashboard, and favourite them again when necessary.
I hope this helps!