WordPress.org

I activated the plugin and nothings happen

You need to integrate the plugin in your theme, see install document for more info.

I get PHP errors when the user comes back from Tractis

If you see error messages like this:

Warning: fsockopen() [function.fsockopen]: unable to connect to ssl://www.tractis.com:443 (Unable to find the socket transport "ssl" - did you forget to enable it when you configured PHP?)

You need php_openssl module working on your php installation in order to comunicate with tractis services in https.

Does the plugin change the database model of Wordpress?

Yes, in order to store the user information the plugin makes the next database changes:

ALTER TABLE $prefix+comments ADD tractis_auth_user varchar(50) NOT NULL DEFAULT '0'

ALTER TABLE $prefix+users ADD tractis_auth_lastlogin int(14) NOT NULL DEFAULT '0'

ALTER TABLE $prefix+users ADD tractis_auth_userid varchar(250) NOT NULL DEFAULT '0'