I don't do massive uploads so never had any need to change this, but here where your problem is...

in ADMIN\add_customers_backend.php find this set of code and add your country....
Code:
if ($country == 'uk') {
    $country_id = 222;
    } else if ($country == 'us') {
         $country_id = 223;
    } else if ($country == 'Ireland') {
         $country_id = 223;
    }
Of course, doing it this way means you will need to use phpmyadmin to look up and use the right country code for your database. mine for the US is actually 241, I added the red.... you'll have to figure out what it is for you.... the ID is not the same for each install!