You can use the CSS Form Builder interface to edit the elements in the form and add in the ones that you need. The idea is that you click on edit contents with the form selected. Then in here you click on the group in the fieldset and click edit. You then select the line you want the element to appear in or create a new line and add the element. Once you have added the element you can change it's type with the select list. You can choose individual checkboxes or a checkbox group.
This info is covered in the Getting Started Guide for CSS Form Builder here:
You will want to check out the section titled Customizing form elements and modifying validation.