From Wiki

If you are using MediaWiki not integrated with vBulletin (through vbWiki) most settings are configured by editing the file LocalSettings.php found in your wiki's installation directory.

For a more detailed list of settings that control MediaWiki's operation refer to LocalSettings.php at Meta

[edit] Tips

To add a new setting to your LocalSettings.php, open the file in a text editor, and add your settings towards the end of the file, but always before the closing ?>.

Also, care should be taken to never add any spaces or new lines after the closing ?>