When you set the Question type to 'Summative scale (Likert)', you will get an empty table with the 'Labels' column on the left, and by default five classes:
To change the column headers, click somewhere in the column, and then on the button. You can also double-click on the column header itself. This will make a text field appear, where you can modify the header of this column.
If you want to add a class (column), click in the column to the right of where you want to insert it. 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.