Students cannot see homework sets until you assign the sets to them. In fact, there are two things that must happen before a student can see a set:
If those two are true, then the student can see that the set exists. If, further, the current date is after the Open Date of the set, then the student can view the set.
To assign students to a set, do the following:
Sets are visible by default. You can make sets invisible from the Homework Sets Editor.