You can do this by going to top tab Schedule, clicking on the section we want to add a student to, then clicking on the subtab Roster.  Once there, click on Options > Add.
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The below will pop up.  You want the Schedule Mode to be "Pull" and the Students to be "Selection".  
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Clicking on "Selection" will automatically bring up a student list window. Search for the correct student and check off their name, then click OK:




After you click OK, the original popup will show the Selected Students to be 1, which is the student you have checked off.  Click OK to add the student.
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