ACF5 no longer adds the default options page that you’re used to be added to the admin menu. If that’s the option page you’re trying to alter the settings of then you have to switch to acf_add_options_page() (http://www.advancedcustomfields.com/resources/acf_add_options_page/) or acf_add_options_sub_page() (http://www.advancedcustomfields.com/resources/acf_add_options_sub_page/)
