here's where I put it
<input type="hidden" name="WA_Store_Cart_1_ID_Add" id="WA_Store_Cart_1_ID_Add" value="<?php echo $row_WADAProducts["ProductID"]; ?>" />
<p class="ATC_Button_Wrapper">
<input type="submit" value="Add to Cart" name="WA_Store_Cart_1_ATC" class="ps_buttons submit" /></p>
<?php }