I downloaded all the zip package on the 13th of March and it seems there are only the installation files for J 1.5 and 1.6.
I also don't understand whether the extension would link the social button +1 to each page of the website or every +1 would count for the domain (preferred).
Thanks for the review. The 1.6 package will install with no problem on 2.5, that's pretty standard for an extension intended for the 1.6 or newer versions of Joomla. This +1 is intended to be a +1 per page, not per domain.