I've integrated FCKeditor 2.6.4 with my new mediawiki 1.14.0
Actually this is very simple to integrate the FCKeditor extension with mediawiki but i can't find any tutorial that describe more detail about how to integrated this clearly.

I assume you already have mediawiki installed on the server and running.

Download FCKeditor extensions for Mediawiki Latest Version

Login into your server

[code]#cd /home/mediawiki/public_html
#wget http://mediawiki.fckeditor.net/nightly/svn/mediawiki_fckeditor_ext_N.tar.gz
#tar -zxvf mediawiki_fckeditor_ext_N.tar.gz[/code]

Rename fckeditor directory

[code]#cd /home/mediawiki/public_html/extensions/FCKeditor
#mv fckeditor fckeditor-old[/code]

Download FCKeditor 2.6.4 latest version

[code]#cd /home/mediawiki/public_html/extensions/FCKeditor
#http://biznetnetworks.dl.sourceforge.net/project/fckeditor/FCKeditor/2.6.4/FCKeditor_2.6.4.tar.gz
#tar -zxvf FCKeditor_2.6.4.tar.gz[/code]

This will automatically created new fckeditor folder.

Configure LocalSettings.php

[code]#cd /home/mediawiki/public_html
#pico LocalSettings.php

#### Add few lines below in the end of the script before ?>

require_once("extensions/FCKeditor/FCKeditor.php");

$wgFCKUseEditor = true;  // When set to 'true' the FCKeditor is the default editor.
$wgUseAjax = true;[/code]

You're done.
Just try to access your media wiki from firefox and try to edit your content.
It will shows FCKeditor wysiwyg editor.