Sorry you had problems with this, - all you needed to do was insert a checkbox in plain old html. (The difficulty was in untangling it from the rest!).
To insert any new form field, put your cursor on the page where you want it, - keeping it out of the way of other code 'entanglements', and have Dreamweaver inset it. So for your checkbox, all that had to go in was:
<label>Join our Mailing List
<input name="checkbox1" type="checkbox" id="checkbox1" value="Yes Please!" />
I am not keen on CSS Form Builder, I must admit, and wouldn't use it to make forms; (seems like overkill on the grouping and formatting side of things and there can be problems hust getting it to fit), but I use the validations and 'show if' behaviours so am quite happy.
Hope you have a smoother ride from here on.