// return array with checkbox elements.
var aryChkbxs = Dom.get('frmPageGrps').elements['chkPageGrps'];

// loop through checkboxes



// loop through page groups
for (var i = 0, len = json.rows.length; i < len; ++i) {
alert(json.rows[i].PermGrpName);

HOW TO SET IF the value of the checkbox matches the value in the above alert set checked = yes
}