Please try following patch and add a checkbox with a 'required' attribute.
diff --git a/PFBC/Element/Checkbox.php b/PFBC/Element/Checkbox.php
index eafd378..1626041 100644
--- a/PFBC/Element/Checkbox.php
+++ b/PFBC/Element/Checkbox.php
@@ -24,7 +24,7 @@ class Element_Checkbox extends OptionElement {
$value = $this->getOptionValue($value);
echo '<label class="', $labelClass, '">';
- echo '<input id="', $this->_attributes["id"], '-', $count, '"', $this->getAttributes(array("id", "class", "value", "checked", "required")), ' value="', $this->filter($value), '"';
+ echo '<input id="', $this->_attributes["id"], '-', $count, '"', $this->getAttributes(array("id", "class", "value", "checked")), ' value="', $this->filter($value), '"';
if(in_array($value, $this->_attributes["value"]))
echo ' checked="checked"';
echo '/> ', $text, ' </label> ';