It is very easy to install the tracking code for your goal into Zen Cart.
A detailed explanation from Zen Cart is here http://tutorials.zen-cart.com/index.php?article=399
<script>
<!--
var value="<?php echo $order_summary['order_total']; ?>"; //Value of sale
var ref="<?php echo $order_summary['order_number']; ?>"; //Your unique reference
var mid="1234";
var goalid="1234";
//-->
</script>
<script src="https://aflite.co.uk/js/track.js"></script>