Today, I got SyntaxHighlighter plugin work on my wiki (for Salesforce Ruby project) http://wiki.are4.us

I follow the instruction.

After installing the Plugin, simply create the http://your-site/wiki/index.php?title=MediaWiki:Geshi.css page. Add the source. Mediawiki uses special pages to control the CSS and layout. There is no need to touch the filesystem and add the Geshi.css via commandline.