How do I add a conditional Sales Tax based in a custom field I added at: "admin/commerce/customer-profiles/types/billing/fields"?

I found this screencast which shows how to create a conditional tax based in the address field: http://www.drupalcommerce.org/videos/tutorials/drupal-commerce-condition...

I know I have to create a condition, but I don't know which is the Data Selector to my custom field.