In order to add a class and an alternate:
Find the class you want. Do not hit the plus sign yet. Look to the right and you will see it says “no alternate”. Click on “no alternate” and a drop down menu will appear with classes. Highlight the class you want as the alternate. Once you have done that you can hit the plus sign to add the class and the alternate to your schedule. Do not hit the finalize button until you have added all the classes you want to add. Once you hit the finalize button you can no longer add or change your classes.