Default country for ads

I'm trying to change the default country in ads from Argentina to United states, based on this post:

http://www.boonex.com/forums/?action=goto&my_threads=1#topic/Default-country-at-new-groups.htm

I've edited modules/boonex/ads/classes/BxAdsModule.php to this:

                    'caption' => _t('_bx_ads_caption_country'),
                    'values' => $aCountries,
                    'value' => 'US',
                    'value' => $sCountry,

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It works fine in groups, but not in ads, and I don't see why not. Does anyone know how this can be changed?