When you go to "add materials" click on the EdPuzzle icon. This will allow you to locate your video on your SM EdPuzzle account and assign it to your class. You have to then go back to the assignment and click the "edit" button on the left hand side and check the "enable grading" box and then put in the point value and due date.
If you don't do this last step, your content will not appear for students.