Magento – Custom customer attribute not processing on onepage checkout registration form

I have followed a tutorial and created a custom customer attribute which I have added to the registration form from the file: persistent/customer/form/register.phtml

I have this code to process the new attribute:

<li>
    <label for="job_title" class="required"><em>*</em><?php echo $this->__('Job Title') ?></label>
    <div class="input-box">
        <input type="text" name="job_title" id="job_title" value="<?php echo $this->escapeHtml($this->getFormData()->getJobTitle()) ?>" title="<?php echo $this->__('Job Title') ?>" class="input-text required-entry" />
    </div>
</li>

This works fine and updates the attribute when the form is submitted. I am however trying to do the same on the onepage checkout registration form. I have edited the file: onepage/billing.phtml

and added the same code, however this breaks the checkout. I believe the error to be caused by this line:

<?php echo $this->escapeHtml($this->getFormData()->getJobTitle()) ?>

I am unsure how to check the right functions for this to work. I have looked at the sql file for the module which created the modules and checked that this attribute can be used in the relevant forms:

$used_in_forms=array();

$used_in_forms[]="adminhtml_customer";
$used_in_forms[]="checkout_register";
$used_in_forms[]="customer_account_create";
$used_in_forms[]="customer_account_edit";
$used_in_forms[]="adminhtml_checkout";
    $attribute->setData("used_in_forms", $used_in_forms)
    ->setData("is_used_for_customer_segment", true)
    ->setData("is_system", 0)
    ->setData("is_user_defined", 1)
    ->setData("is_visible", 1)
    ->setData("sort_order", 100);

$attribute->save();

How can I get this custom customer attribute to display on the onepage registration form? This is a Customer and not Customer Address attribute. Any help is much appreciated.

UPDATE

Below is the code which I have used in the module.

This is the script which is in the sql directory of the module (here there are other custom attributes which I created but this is the code for the "job_title" attribute:

$installer = $this;
$installer->startSetup();


$installer->addAttribute("customer", "job_title",  array(
    "type"     => "varchar",
    "backend"  => "",
    "label"    => "Job Title",
    "input"    => "text",
    "source"   => "",
    "visible"  => true,
    "required" => false,
    "default" => "",
    "frontend" => "",
    "unique"     => false,
    "note"       => ""
));

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

$used_in_forms=array();

$used_in_forms[]="adminhtml_customer";
$used_in_forms[]="checkout_register";
$used_in_forms[]="customer_account_create";
$used_in_forms[]="customer_account_edit";
$used_in_forms[]="adminhtml_checkout";   

$attribute->setData("used_in_forms", $used_in_forms)
    ->setData("is_used_for_customer_segment", true)
    ->setData("is_system", 0)
    ->setData("is_user_defined", 1)
    ->setData("is_visible", 1)
    ->setData("sort_order", 100);

$attribute->save();

The "Helper" directory has this code in a file called "Data.php":

<?php
class Mymodule_Customerattributes_Helper_Data extends Mage_Core_Helper_Abstract
{
}

The "config.xml" file has this:

<?xml version="1.0"?>
<config>
  <modules>
    <Mymodule_Customerattributes>
      <version>0.1.0</version>
    </Mymodule_Customerattributes>
  </modules>
  <global>
    <helpers>
      <customerattributes>
        <class>Mymodule_Customerattributes_Helper</class>
      </customerattributes>
    </helpers>
    <models>
      <customerattributes>
        <class>Mymodule_Customerattributes_Model</class>
        <resourceModel>customerattributes_mysql4</resourceModel>
      </customerattributes>
    </models>
    <resources>
      <mymodule_customerattributes_setup>
        <setup>
          <module>Mymodule_Customerattributes</module>
          <class>Mage_Customer_Model_Entity_Setup</class>
        </setup>
        <connection>
          <use>core_setup</use>
        </connection>
      </mymodule_customerattributes_setup>
      <mymodule_customerattributes_write>
        <connection>
          <use>core_write</use>
        </connection>
      </mymodule_customerattributes_write>
      <mymodule_customerattributes_read>
        <connection>
          <use>core_read</use>
        </connection>
      </mymodule_customerattributes_read>
    </resources>
  </global>
</config> 

Some names such as the module name has been changed as it contained my employer's company name. The structure is the same though.

Thank you.