Step 1: Create /app/code/local/Magemonkeys/Customerattribute/sql/magemonkeys_myattribute_setup/install-0.1.0.php file and add the following code in this file:

<?php
$installer = $this;

$installer->startSetup();

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$entityTypeId     = $setup->getEntityTypeId('customer');
$attributeSetId   = $setup->getDefaultAttributeSetId($entityTypeId);
$attributeGroupId = $setup->getDefaultAttributeGroupId($entityTypeId, $attributeSetId);

$setup->addAttribute("customer", "myattribute",  array(
    "type"     => "varchar",
    "backend"  => "",
    "label"    => "My Attribute",
    "input"    => "text",
    "source"   => "",
    "visible"  => true,
    "required" => false,
    "default" => "",
    "frontend" => "",
    "unique"     => false,
    "note"       => "My Attribute"
));

$attribute   = Mage::getSingleton("eav/config")->getAttribute("customer", "myattribute");

$setup->addAttributeToGroup(
    $entityTypeId,
    $attributeSetId,
    $attributeGroupId,
    'myattribute',
    '1000'
);

$usedIn=array();

$usedIn[]="adminhtml_customer";
$usedIn[]="checkout_register";
$usedIn[]="customer_account_create";
$usedIn[]="customer_account_edit";
$usedIn[]="adminhtml_checkout";
        $attribute->setData("used_in_forms", $usedIn)
                ->setData("is_used_for_customer_segment", true)
                ->setData("is_system", 0)
                ->setData("is_user_defined", 1)
                ->setData("is_visible", 1)
                ->setData("sort_order", 101)
                ;
        $attribute->save();



$installer->endSetup();
?>

Step 2: Create /app/code/local/Magemonkeys/Customerattribute/etc/config.xml and add the following code in this file:

<?xml version="1.0"?>
<config>
	<modules>
		<Magemonkeys_Customerattribute>
			<version>0.1.0</version>
		</Magemonkeys_Customerattribute>
	</modules>
	<global>

		<resources>
			<Magemonkeys_Myattribute_setup>
				<setup>
					<module>Magemonkeys_Customerattribute</module>
					<class>Mage_Customer_Model_Entity_Setup</class>
				</setup>
				<connection>
					<use>core_setup</use>
				</connection>
			</Magemonkeys_Myattribute_setup>
			<Magemonkeys_Myattribute_write>
				<connection>
					<use>core_write</use>
				</connection>
			</Magemonkeys_Myattribute_write>
			<Magemonkeys_Myattribute_read>
				<connection>
					<use>core_read</use>
				</connection>
			</Magemonkeys_Myattribute_read>
		</resources>
	</global>

</config>

Step 3: Create app/etc/modules/Magemonkeys_Customerattribute.xml and add the following code in this file:

<?xml version="1.0"?>
<config>
	<modules>
		<Magemonkeys_Customerattribute>
			<active>true</active>
			<codePool>local</codePool>
		</Magemonkeys_Customerattribute>
	</modules>
</config>