The installation of most extensions in MediaWiki consists of downloading the extension's archive and extracting it in the extensions folder within your MediaWiki installation. Then you can enable the extension by simply adding a line in the LocalSettings.php file in the following manner:
include("extensions/YourExtensionName.php");
Generally, each extension should have a Readme file that will explain if it needs any special actions taken before the installation.
In the MediaWiki Extensions section you will also find:
