On the 1.2.5 release only the Effervescence+ theme provides an implementation of this plugin. But if you wish to try the nightly build, now all the distributed themes have the support built in.
Otherwise you will need to create a registration page script and place a call on the `printRegistrationForm()` function. The correct documentation for using the plugin is http://www.zenphoto.org/documentation/plugins/_plugins---register_user.php.html. The reference you quote above is for the plugin's option handler class which you do not need to concern yourself about.
Documentation for Zenphoto functions is automatically generated. That process documents all functions including ones that are internally used. Generally, the link on the plugin page will take you to the correct documentation.