http://meta.wikimedia.org/wiki/MediaWiki_FAQ#How_do_I_add_the_sandbox_functionality_to_my_installation_of_the_wiki.3F

First you need to go to the "special pages" link. Then click on "All system messages". Find and click on "nogomatch". click on "Edit this page". replace the existing text with the following:

Place wikitext similar to the following in MediaWiki:Nogomatch:

There is no page called $1. You can [[$1|create this page]].

This will create a regular "redlink" in the search results page, leading to the edit page. Your users can then create the page as usual.

Edit the file: Language.php, which locates the directory of languages.

documented on: 2007.04.20