try this change:
line 1 to:
<?php if(isset($_POST['Activity'])) $_POST['Activity'] = implode(" ", $_POST['Activity']); ?>
and line 33 to:
$WADbSearch1->keywordComparison($KeyArr2,"".((isset($_POST["Activity"]))?$_POST["Activity"]:"") ."","AND","=",",%20","%20","%22","%22",0);