There are a few reasons why this could be the case: 

  1. You are looking for it on your dashboard, and have not added it to your dashboard. If you click on Courses on the left, then "All courses" you'll see everything you are registered for. You can star the courses you'd like to appear on your dashboard, and unstar the ones you don't want to see.
  2. If you literally just registered for a course, it may be a few hours before it is available to you on Canvas.
  3. Your instructor has to choose to use Canvas. Not all courses will use Canvas.
  4. Your instructor has to publish the course for you to see it. Sometimes courses are not immediately published while an instructor prepares the course.