CiviCRM in Wordpress
Basically you need to :


Download the language files from SF (it seems you have already done this)
Unpack the language-tarball in wp-content/plugins/civicrm. it creates subdirectories civicrm/i10n and civicrm/l18n with several files 
Login to CiviCRM and Go to Administer » Localization » Languages, Currency, Locations
Enable Portuguese as default language


Beleza, should be working!

More details can be found here:
https://wiki.civicrm.org/confluence/display/CRMDOC/i18n+Administrator's+Guide:+Using+CiviCRM+in+your+own+language