If you are trying to add a custom tab to the product add/edit page in Magento admin, then you are at the right place. Today I will tell you a damn simple method to add the custom tab to product add/edit page. We will have to add few lines to the xml layout file of your custom module. Then create a phtml file that contains the html part to be shown in the tab.

Here is the detailed description of the process:
1. Create a directory named ‘custom’ at app\design\adminhtml\default\default\template location.

2. Create file named content.phtml in the above directory so that the full path looks like app\design\adminhtml\default\default\template\custom\content.phtml

3. In content.phtml you can write any php and html code according to your need.

<?php
//Get the current product
$product = Mage::registry('product');
?>

<div>
    <div>
        <h4>Custom Tab</h4>
    </div>
    <div id="group_fields4">
 <?php
 var_dump($product);
 ?>
    </div>
</div>

4. Create the block file in app\code\local\<namespace>\<module_name>\Block\Adminhtml\Config.php. Add following content to the file:

<?php
class <namespace>_<module_name>_Block_Adminhtml_Config extends Mage_Core_Block_Template implements Mage_Adminhtml_Block_Widget_Tab_Interface
{
 public function __construct(){
  $this->setTemplate('custom/content.phtml');
  parent::__construct();
 }

 //Label to be shown in the tab
 public function getTabLabel(){
  return Mage::helper('core')->__('Custom Tab');
 }

 public function getTabTitle(){
  return Mage::helper('core')->__('Custom Tab');
 }

 public function canShowTab(){
  return true;
 }

 public function isHidden(){
  return false;
 }
}

5. Open the app\design\adminhtml\default\default\layout\<module_name>.xml. Add following lines to this file:

<!-- To add a tab on new product page -->
<adminhtml_catalog_product_new>
 <reference name="product_tabs">
  <action method="addTab">
   <name>custom_tab</name>
   <block template="custom/content.phtml">module_name/adminhtml_config</block>
  </action>
 </reference>
</adminhtml_catalog_product_new>
<!-- To add a tab on new product page -->

<!-- To add a tab on edit product page -->
<adminhtml_catalog_product_edit>
 <reference name="product_tabs">
  <action method="addTab">
   <name>custom_tab</name>
   <block template="custom/content.phtml">module_name/adminhtml_config</block>
  </action>
 </reference>
</adminhtml_catalog_product_edit>
<!-- To add a tab on edit product page -->