Or you can directly download the source from https://launchpad.net/mediawiki-el .
After you install the package from AUR, open your emacs configuration file (usually, Template:Filename) and add the following line in it.
Then add the mediawiki sites to the configuration file. For example, the following line adds ArchWiki to it.
(setq mediawiki-site-alist (append '("ArchWiki" "https://wiki.archlinux.org/" "username" "password" "Main Page") mediawiki-site-alist))
Where Archwiki is the name for the site. https:/wiki.archlinux.org/ is the url of the site and username and password is your username and password of this site. Main Page is the default page opened when you connect to the site.