Click 'OK' on the configuration changes. Zend changing your php.ini file location so its better to make a note of this. And then you should see the finished successful installation screen.
Restart the Apache and check whether the Zend Optimizer is running.
After 'php -V' You should see similar to this message depending to the versions on your server:
# php -v
PHP 5.1.6 (cli) (built: Feb 23 2007 10:11:28)
Copyright (c) 1997-2006 The PHP Group
Zend Engine v2.1.0, Copyright (c) 1998-2006 Zend Technologies
with the ionCube PHP Loader v3.1.16, Copyright (c) 2002-2006, by ionCube Ltd.
With Plesk, by default there is a Ioncube loader installed that must be disabled.
This can be done as commenting the 'zend extension' in this file:
'/etc/php.d/ioncube-loader.ini'