Error: Fatal error: Call to undefined function wp_registration_url() in /home/sugarpop/public_html/

I've downloaded Pay Per VIew. Error on installation reads:
Fatal error: Call to undefined function wp_registration_url() in /home/sugarpop/public_html/wp-content/plugins/pay-per-view/pay-per-view.php on line 213

Deactivating the plugin will bring my site back up. Activating it will cause this error. I've called my server host. Multibyte string PHP Extension is activated.

Not sure what to do now?

Nicole