In a Silva instance, you cannot mix PAS acl_user and regular Zope 2 one.
This extension require at least Silva 3.0 or higher. For previous release of Silva, please use previous versions of silva.pas.base.
In Silva 3.0, it is installed by default.
You can find the code of this extension in Git: https://github.com/silvacms/silva.pas.base
License: New BSD