How do I add additional states for a country?

Open jigoshop_countries.class.php

Find:

    'YT' =>  'Yukon Territory'
),

Under it add:

'NZ' => array(
    'AA' =>  'Region One' ,
    'BB' =>  'Region Two'
),

This will add something like this to your Checkout page (and wherever else countries are displayed):

image