Assignments must be published for students to view them. This "locks down" the assignment and generates randomized versions for each student based on the randomization settings you specified. For more information about controlling randomization click here: Randomization within Assignments.
Click "Publish to Class" in the left-hand menu, followed by "Publish Assignment" at the bottom of the publish page. You also have the option to generate student PDFs, giving you a printable versions of learning module slides. The date and time the assignment was published will be shown in the header of the assignment page.
The left hand course menu will also change to include additional options for the published assignment and the link to publish the assignment will disappear.