Hi Samuele -
You are looking at the "latest release" - which includes EAP releases, possibly, now.
Need to look carefully at the versions being displayed.
The latest is showing a beta version for JIRA5.0 which is also in Beta. As follows, for Greenhopper at the moment.
Plugin Details<dl><dt>Latest Version:</dt><dd>5.9 beta</dd><dt>Availability:</dt><dd>JIRA 5.0 beta</dd><dt>Plugin System:</dt><dd>Two</dd><dt>Install Location:</dt><dd>jira-home/plugins/installed-plugins</dd><dt>State:</dt><dd>Beta</dd><dt>License:</dt><dd>Commercial</dd><dt>Supported By:</dt><dd>Unsupported</dd></dl>
GreenHopper is supported. It should be downloaded from here, http://www.atlassian.com/software/greenhopper/pricing.jsp.
I believe that it is also built into the latest release of JIRA, so it doesn't need a separate install any more