In edit mode, click the "Input" tab. You'll then see an "Add Student" button, from which you can manually add a student to your assessment. Note that if you're adding students that are enrolled in your class, you should use the "Sections" menu to choose the relevant sections. If students are not showing up in sections that they should be in, this represents a scheduling issue in your SIS. Instead of continually adding students manually, contact the school admin in charge of your SIS and ask them to update enrollments.