Admin Tools 2.5.7 and later no longer includes the Joomla! Updater feature and the Joomla! update icon notification plugin. As the core Joomla! Update component and icon notification plugin, written by our team and donated to the Joomla! project, have a very slow release cycle we decided to release an improved Joomla! update component called Akeeba CMS Update. It is available free of charge. It is still considered alpha quality software as we cannot work around Joomla!'s post-installation script which is bound to fail on low-end shared hosts. Unfortunately this is a problem with Joomla! itself and it prevents us from providing a smooth update experience to all of our users. We know what is wrong and how to fix it but that requires changing Joomla! itself. Sorry!
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.