==This lines work for me (Mediawiki 1.11.0)==
Just Add
// change directory >> /usr/local/src/geshi/ << according to your
geshi installation
include_once('/usr/local/src/geshi/geshi.php');
$languagesPath = "/usr/local/src/geshi/geshi";
to the top of '''SyntaxHighlight_GeSHi.php'''