i made 2 changes to fix the problem.
i changed 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",",","%22","%22",0);