After enabling One Page Checkout plugin, you have to manage settings in NOP admin.
path: /Admin/OnePageCheckout/Configure
Select tab: Attributes
Enabled is adding additional fields to customer address form.
When adding new form attribute, user can configure following fields:
Code - attribute code/name;
Widget Zone - widget zone, where to display attribute.
Possible widget zones:
before customer type option
after customer type option
before newsletter
after newsletter
Show for business - check if you want to display attribute on business customer form;
Show for private - check if you want to display attribute on private customer form;
Display order - display order of added attributes.