Download and extract the file(s) in a directory called OpenSearchXml in your extensions/ folder.
Add the following code at the bottom of your LocalSettings.php:
require_once"$IP/extensions/OpenSearchXml/OpenSearchXml.php";$wgEnableMWSuggest=true;#Not needed if using MediaWiki 1.20+
Done! Navigate to "Special:Version" on your wiki to verify that the extension is successfully installed.
This extension is being used on one or more Wikimedia projects. This probably means that the extension is stable and works well enough to be used by such high-traffic websites. Look for this extension's name in Wikimedia's CommonSettings.php and InitialiseSettings.php configuration files to see where it's installed. A full list of the extensions installed on a particular wiki can be seen on the wiki's Special:Version page.