you have hand coding in place that is breaking the ability to use the behavior.
on the update page,
change:
$KeyArr1 = array("Itinerary");
to:
$KeyArr1 = array("Itinerary");
$KeyArr2 = array("ActivityID");
and change:
$WADbSearch1->addComparisonFromList("ActivityID","Activity","AND","=",0);
to:
$WADbSearch1->keywordComparison($KeyArr2,"".((isset($_POST["Activity"]))?$_POST["Activity"]:"") ."","AND","Includes",",%20","%20","%22","%22",0);
and add this code at line 1:
<?php if(isset($_POST['Activity'])) $_POST['Activity'] = implode(", ", $_POST['Activity']); ?>