Magento - add an attribute option in setup script

install-1.0.0.0.php

<?php

$installer = $this;

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$attr_model = Mage::getModel('catalog/resource_eav_attribute');
$attr = $attr_model->loadByCode('customer', $attribute_name);
$attr_id = $attr->getAttributeId();

$option = array('attribute_id' => $attr_id, 'values' => array(1000 => $option_label));

$setup->addAttributeOption($option);

$installer->endSetup();