you can add new row after specific content directly to your email template to get Discount amount from order object as below
<table>
<tr>
<td>Total discount </td>
<td> {{var order.getBaseDiscountAmount()}}</td>
</tr>
</table>
Above code is just concept to place discount code directly in email template from admin
Or if you wish to add in any phtml like in total.phtml
you can with
<?php echo Mage::helper('core')->currency($order->getBaseDiscountAmount(), true, false);?>
hope this will work for you.