Overriding a core admin block not working - to add carriers

Question

I am new to Magento and i'm trying to override Mage/Adminhtml/Block/Sales/Order/Shipment/Create/Tracking in a module to add extra options for carriers in the shipping and tracking section and i cant figure out why this is not working?

Heres my code:

Module xml: app/etc/modules/Essential_Emailtracking.xml

<?xml version="1.0" encoding="UTF-8"?>
<config>
 <modules>
  <Essential_Emailtracking>
   <active>true</active>
   <codePool>local</codePool>
  </Essential_Emailtracking>
 </modules>
</config>

Module config.xml: app/code/local/Essential/Emailtracking/etc/config.xml

<?xml version="1.0" encoding="UTF-8"?>
 <config>

  <modules>
   <Essential_Emailtracking>
     <version>0.1.0</version>
   </Essential_Emailtracking>
  </modules>

  <global>
   </blocks>
    <adminhtml>
     <rewrite>
      <sales_order_shipment_create_tracking>Essential_Emailtracking_Block_Sales_Order_Shipment_Create_Tracking</sales_order_shipment_create_tracking>
     </rewrite>
    </adminhtml>
   </blocks>
  </global>
 </config>

My Block to override core mage block: app/code/local/Essential/Emailtracking/Block/Sales/Order/Shipment/Create/Tracking.php

<?php

class Essential_Emailtracking_Block_Sales_Order_Shipment_Create_Tracking extends Mage_Adminhtml_Block_Sales_Order_Shipment_Create_Tracking
{
/**
 * Prepares layout of block
 *
 * @return Mage_Adminhtml_Block_Sales_Order_View_Giftmessage
 */
protected function _prepareLayout()
{
    $this->setChild('add_button',
        $this->getLayout()->createBlock('adminhtml/widget_button')
            ->setData(array(
                'label'   => Mage::helper('sales')->__('Add Tracking Number'),
                'class'   => '',
                'onclick' => 'trackingControl.add()'
            ))

    );

}

/**
 * Retrieve shipment model instance
 *
 * @return Mage_Sales_Model_Order_Shipment
 */
public function getShipment()
{
    return Mage::registry('current_shipment');
}

/**
 * Retrieve
 *
 * @return unknown
 */
public function getCarriers()
{
    $carriers = array();
    $carrierInstances = Mage::getSingleton('shipping/config')->getAllCarriers(
        $this->getShipment()->getStoreId()
    );
    $carriers['usps2'] = Mage::helper('sales')->__('USPS');
    $carriers['canada_post'] = Mage::helper('sales')->__('Canada Post');
    $carriers['aus_post'] = Mage::helper('sales')->__('Australia Post');
    $carriers['custom'] = Mage::helper('sales')->__('Custom Value');
    foreach ($carrierInstances as $code => $carrier) {
        if ($carrier->isTrackingAvailable()) {
            $carriers[$code] = $carrier->getConfigData('title');
        }
    }
    return $carriers;
  }
}

Answer 1

You have an error in your config.xml. Instead of opening blocks tag, you have closed it. Correct it as below:

<?xml version="1.0" encoding="UTF-8"?>
 <config>

  <modules>
   <Essential_Emailtracking>
     <version>0.1.0</version>
   </Essential_Emailtracking>
  </modules>
<global>
       <blocks>
        <adminhtml>
         <rewrite>
          <sales_order_shipment_create_tracking>Essential_Emailtracking_Block_Sales_Order_Shipment_Create_Tracking</sales_order_shipment_create_tracking>
         </rewrite>
        </adminhtml>
       </blocks>
      </global>
     </config>