This version requires PHP 5.3, 5.4 or 5.5. The rationale behind this is explained in our statement of mid-February 2013. It won't install on hosts running PHP 5.2 or earlier. PHP 5.3.4 or later is required, due to show-stopper bugs in earlier versions of PHP 5.3. Moreover, due to the necessary Joomla! API changes found only in Joomla! 2.5.6 or later, this version will not install on Joomla! 2.5.5 or earlier versions. In any case, it will tell you exactly why it cannot be installed (minimum PHP or Joomla! version not satisfied).
This version of our software can be installed on Joomla! 2.5.6 or any later 2.5.x release, as well as any 3.2.x or 3.3.x release. It is not compatible with Joomla! 3.0 or 3.1. These versions of Joomla! are old and vulnerable. If you are using them please upgrade to the latest Joomla! 3 release available from Joomla.org.