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.
Other sites can also be added by append the configuration list to the list variable mediawiki-site-alist.