the select list for the category is named Parent:
<select name="Parent" id="Parent" onchange="WA_FilterAndPopulateSubList(Child_WAJA,MM_findObj('Parent'),MM_findObj('Child'),0,0,false,': ')">
your code to set the session:
$_SESSION["CategoryValue"] = "".((isset($_POST["CategoriesFullID"]))?$_POST["CategoriesFullID"]:"") ."";
refers to a form element named CategoriesFullID which does not exist on the page, it should be edited to use the Parent form element instead.