I am using MediaWiki version 1.7 for the Wikipedia mirror. However, when I copied the "essential" extensions Cite.php and ParserExtensions.php and enabled them both in LocalSettings.php, I am seeing a lot of errors. However, when I disable
Cite.php, then the errors disappear, but <ref.../ref> no longer work.
Download this version and you will find it works and is included and integrated.
ftp://ftp.wikigadugi.org/wiki/MediaWiki/
And if that doesn't work, probably best to give us full details of the errors you're seeing (exact error message, when it happens, changes you've made to LocalSettings, link to a one or two other problems of same type from google, a URL to your wiki if it's public, etc). Someone on the list is likely to have seen it before, or have a hunch what the problem is, but the details will probably be what triggers that memory.
All the best, Nick.