Adding custom special fields

Hi, I am following the lesson 3 described in http://docs.woothemes.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/ about how to add a custom special field but only works for me the first step.

The new field is displayed in the checkout page but it does not work the validation and the new field is not displayed in the order edition page.

Any idea why?

https://wordpress.org/plugins/woocommerce/