First of all, you have to create a new module. After that, you have to put the code in config.xml

 <adminhtml>
        <events>    
            <adminhtml_block_salesrule_actions_prepareform>
                <observers>
                    <magemonkeys_custofield>
                        <type>model</type>
                        <class>Magemonkeys_Custofield_Model_Observer</class>
                        <method>adminhtmlBlockSalesruleActionsPrepareform</method>
                    </magemonkeys_custofield>
                </observers>
            </adminhtml_block_salesrule_actions_prepareform>
        </events>
 </adminhtml>

in Model/Observer.php file you have to put this method and code.

<?php
class Magemonkeys_Custofield_Model_Observer
{
    public function adminhtmlBlockSalesruleActionsPrepareform($observer)
    {

        $fieldset = $observer->getForm()->getElement('action_fieldset');
        $fieldset->addField('sku', 'text', array(
            'name' => 'sku',
            'label' => Mage::helper('freeproduct')->__('Add skus'),
            'title' => Mage::helper('freeproduct')->__('Add skus'),
            'note' => Mage::helper('freeproduct')->__('Add sku(s) separated with comma'),
        ));
    }
}
?>

And after that, you have to flush the cache and check.