To configurate the module, click on Addons → Next Due Bonus. The page has serveral informationboxes which explains how to configure the module.
After configuration is done, the bonus should be visible during ordering.
Multilanguage Support
Module does support multiple languages.
To add a new language, just open the directory /modules/addons/extendnextdue/lang/, create a copy of the english language file and rename it to the wished language.
The variables inside of the language file are pretty self explaining.