If you're reading this, then you're already logged in, so I'll keep this brief.
If you ever go to a lesson and it says it's locked and that you need to Enroll, you're not logged in. Solution: Log in!
If you ever click on a course and don't see the curriculum page, you're not logged in. Solution: Log that thang in.
If you ever get a 404 Error that doesn't necessarily mean you aren't logged in. You just followed a link that is no longer correct. Logging in won't hurt. Or click the logo in the Menu bar to go back to the front page of the Classroom.