UPDATE
Please have look here for a updated version.
Note: its about Twig-extensions not about the former “Silex Twig Extension”, this now called TwigServiceProvider.
First fetch the Twig-extensions code and put them in your vendor dir:
git submodule add git://github.com/fabpot/Twig-extensions.git vendor/Twig-extensions/fabpot git submodule init git submodule update
Then register the autoloader by using registerPrefixes:
$app['autoloader']->registerPrefixes(array( 'Twig_Extensions_' => array(__DIR__.'/../vendor/Twig-extensions/fabpot/lib')));