Changing Slugs is not working properly.

Greetings..
I read your manual:
USTOMIZE YOUR ROOT SLUG

If you want to change the Slug, or url, of your Questions you can define any of the following in your wp-config.php

QA_SLUG_ROOT

QA_SLUG_ASK

QA_SLUG_EDIT

QA_SLUG_UNANSWERED

QA_SLUG_TAGS

QA_SLUG_CATEGORIES

QA_SLUG_USER

using the following format define(‘QA_SLUG_ROOT’, ‘yourslughere’);

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Im actually doing that and its not working.
I set it as:
define(‘QA_SLUG_ROOT’, ‘preguntas’);

And when I open http://www.cerebrodigital.org/preguntas It wont open the plugin.
Not sure what is wrong.
Thanks in advance.