1) The checkbox name needs to end in "[]"
<td width="5%"><input name="ATCCheck[]" type="checkbox" id="ATCCheck" value="<?php echo $row_Recordset1['id']; ?>" /></td>
2) The default where clause should be 0 != 0 and the trigger should be the chkbox:
change:
$WADbSearch1_DefaultWhere = "AND id=0";
if (!session_id()) session_start();
if (isset($_POST["AddToCartButton_x"])) {
to:
$WADbSearch1_DefaultWhere = " 0 != 0";
if (!session_id()) session_start();
if (isset($_POST["ATCCheck"])) {