Overview
Assignments can be viewed in both Forum and Course Builder. Instructor roles and above can access assignment details preview in Forum and Course Builder. Only Authors, Reviewers and Admins can access the assignment editor in Course Builder.
Navigate to the assignment preview
All instructors for a class should have access to the assignment preview. The preview includes all details of the assignment, including notes visible only to the instructor.
To navigate to the preview:
Open the assignment page in Forum
Click on the View Instructor Notes button on the right

Alternatively, if you're already in Course Builder:
Open the Course page of the relevant course
Navigate to the desired assignment
Click on the assignment title or Preview button

Navigate to the assignment editor
The lesson plan editor is only available to users who have an Author or Reviewer role on the course. Otherwise, you can only access the assignment preview (see section above).
To open the assignment editor:
Open the desired course in Course Builder
Navigate to the desired assignment
Click on the assignment title
For more information on how to edit and publish assignments, see this article.