The plugin was merged into the WebFinger plugin.
The latest version of the WebFinger plugin (for WordPress) http://wordpress.org/plugins/webfinger/ drops support for the old and experimental specs, this plugin brings back all this legacy stuff. It will only work in combination with the latest version of the WebFinger plugin (>= 3.0.0): https://github.com/pfefferle/wordpress-webfinger