1. Copy /includes/languages/english/modules/payment/moneyorder.php to /includes/languages/portuguese/modules/payment/moneyorder.php
2. Use a reliable text editor on your PC (such as Notepad++ http://notepad-plus.sf.net ) to edit /includes/languages/portuguese/modules/payment/moneyorder.php ... make your translations, and save the file. When saving the file, use the correct encoding for that language.
3. Upload the /includes/languages/portuguese/modules/payment/moneyorder.php file to your server.