When you set the Question type to 'Multiple choice', you will get an empty table with three classes, called 'Class 1', 'Class 2' and 'Class 3'. To change these column headers, just click somewhere in the column, and then on the button. You can also double-click on the column header. This will make a text field appear, where you can modify the name of this class (column).
If you want to add a class (column), click in the column to the right of where you want to insert it. For instance in the example above, if you want to add a column between Class 2 and Class 3, you should click in the Class 3 column. Then press the button in the toolbar.
If you want to remove a class (column), click in the column itself and then on the button.