Add and show new column in sales_flat_order and sh

I have added a new column (exported)in sales_flat_order and add at files at this location:

app/code/core/Mage/Adminhtml/Block/Sales/Order/Grid.php

protected function _prepareCollection()
{
 $collection = Mage::getResourceModel($this->_getCollectionClass())
    ->addAttributeToSelect('*')
    ->joinAttribute('exported','sales/order','sales_flat_order.entity_id',null,'left');
}

protected function _prepareColumns()
{
 $this->addColumn('exported', array(
            'header' => Mage::helper('sales')->__('Exported'),
            'index' => 'exported',
            'type'  => 'checkbox',
            'name'  =>'exported',
            'value'    =>$this->getExported()==1 ? 'true' : 'false',
          ));
  }

after that it showing on order grid in admin site,but it is not showing value and name,

I am new in magento,so please help me , stuck from 2 days. Thanks for Assistance.

The method _prepareCollection() uses sales_flat_order_grid table as asource, thus you have to add the column to sales_flat_order_grid table and update the values of that column from the appropriate column of sales_flat_order table.

In this case, Magento will automatically update this column in sales_flat_order_grid table for future orders.

The better way to display the boolean column is Yes/No renderer. Use the following code for this in _prepareColumns() method

 $this->addColumn('exported', array(
            'header' => Mage::helper('sales')->__('Exported'),
            'index' => 'exported',
            'type'  => 'options',
            'width' => '70px',
            'options' => array(
                1  => Mage::helper('sales')->__('Yes'),
                0  => Mage::helper('sales')->__('No'),
            ),
        ));

There are some other useful articles about cutomizing order grid. Check out the links below:

http://inchoo.net/ecommerce/magento/how-to-extend-magento-order-grid/
http://www.ecomdev.org/2010/07/27/adding-order-attribute-to-orders-grid-in-magento-1-4-1.html
http://www.demacmedia.com/magento-commerce/mini-tutorial-adding-column-to-orders-grid-in-magento-backend/
http://www.atwix.com/magento/column-to-orders-grid/