How do I enter students' Reserve choices?

Reserve Choices

On the Populations Screen, choose "1" or "2" from the drop-down marked 'Allow a Reserve?' and then click Apply.

An extra Choice box (or 2, if you selected 2 reserves) then appears on the Students' Choices Screen, so that you can enter the Reserve Subject for each student.

And an extra button ('Use Reserves') appears on the main Assignments Screen.
Click the Help button on that screen for more details.

See also:  
-- this article on using the Reserve Choices    and  
-- this article on prioritising Choices.