I'm sorry, I was pretty sure that wiki page mentioned php-domxml, but apparently I was wrong.
PSA seems to have a dependency on the php-domxml which is incompatible with PHP 5.2.6 resulting in a failed install.
AFAIK Plesk doesn't have a dependency on php-domxml. However, there is no php-domxml package in PHP 5. The XML package for PHP 5 is called php-xml. You should be able to remove php-domxml and install php-xml afterwards.
I wouldn't mind writing a full how-to as soon as I can get this resolved
Better idea: update the wiki if it works.