Danny when you create the attribute in the database it is classified as a YORN field which means it will always have a value (0 = No, 1= Yes), so you really cannot make a checkbox "required" as it is never an empty field. What I would do, in in the workflow when they submit the form, check that the value of this checkbox field is = 1, if is not = 1, then display a message which says they need to make sure this field is checked on before they can submit.
This is only one possible solution, there are others as well.
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Stephen Hume
Sheffield Scientific LLC
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