Since version 2.3 AJAX Register is no longer compatible with Joomla 1.6 and 1.7 nor any 2.5 version less than 2.5.6. Dropping support for outdated versions of Joomla was necessary in order to achieve compatibility with Joomla 3.x.
If you use Joomla 1.6 or 1.7 we strongly suggest you upgrade to current version of 2.5 because all older versions are unsupported and insecure.
If you must run 1.6 or 1.7 by some reason you can contact us to send you the latest version of AJAX Register that worked with it. But please note that this version will not be supported and it will not have all features of the latest version for Joomla 2.5.