You can add a new field to the table by either adding a row at the end of the fields list or by inserting a row anywhere in the list.

To add a new field to the end of the fields list, choose Structure|Add Field (or right-click anywhere in the Table designer and choose Add Field from the menu).

To insert a new field between other fields, select a field, and choose Structure|Insert Field, or right-click and choose Insert Field from the context menu. The new field’s row of properties appears above the one you selected.