WordPress.org

1. on Regular Wordpress

. Copy the files to you root plugins folder, so you have a tree like:

.../wp-content/plugins/wp-xajax.php .../wp-content/plugins/wp-xajax/xajax.inc.php .../wp-content/plugins/wp-xajax/...all the other files

. Activate the plugin

1. on Wordpress MU

. Copy the files to you root plugins folder, so you have a tree like:

.../wp-content/mu-plugins/wp-xajax.php .../wp-content/mu-plugins/wp-xajax/xajax.inc.php .../wp-content/mu-plugins/wp-xajax/...all the other files