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.
The below will pop up. You want the Schedule Mode to be "Pull" and the Students to be "Selection".

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.

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.