Create the file /etc/yum.repos.d/atrpms.repo with the following contents:
[atrpms]
name=EL5 $releasever - $basearch - ATrpms
baseurl=http://dl.atrpms.net/el$releasever-$basearch/atrpms/stable
gpgkey=http://ATrpms.net/RPM-GPG-KEY.atrpms
gpgcheck=1
Then run yum search mediawiki to see that you’ve got the ATrpms Repo!
From there it’s just yum install mediawiki