set the redirect URL:
$WA_redirectURL = "../member_update.php?MemberID=".$_SESSION['MemberID'];
to:
$WA_redirectURL = "../member_update.php?MemberID=[Insert_ID]";
notice this section of the code just before the header redirect:
$_SESSION[$WA_sessionName] = mysql_insert_id($WA_connection);
if ($WA_redirectURL != "") {
$WA_redirectURL = str_replace("[Insert_ID]",$_SESSION[$WA_sessionName],$WA_redirectURL);
the first line sets the session, then the 3rd line looks at the redirect URL and replaces [Insert_ID] with the session value.