As you can see, when installing manually you need to do quite a fiew things, and essentially the installer in Plesk just asks you for the data to be used with them, and does them for you.
Info on any additional configuration options should be outlines in the doecumentation section of the website.
Of course it would be nice if there was someone here on ther forum who has recently installed it and remembers what the options were and could summarise them for us here.
i think you need to have an open-xchange server and this plugin only help you to configure the users in ox and do some migration of horde data for you ...