I just manage to install it. Check that you are using the 2.3 install instructions cos mine looks like this....
<!-- AJAX Attribute Manager -->
<?php if ((basename($_SERVER["SCRIPT_NAME"]) == FILENAME_CATEGORIES)) {
require('attributeManager/includes/attributeManagerHeader.inc.php'); }?>
<!-- AJAX Attribute Manager end -->
<head>
Also if you get a stack code after installing. go to attributeManager/languages/English/attributeManager.php. Change the first line '<?' to '<?php'