In case you want to use the full apps repo within your ownCloud installation (most probably for development)
you can follow these steps:

1. clone this repo into your ownCloud installation into a new folder (maybe called apps2):
   git clone git@github.com:owncloud/apps.git apps2

2. add this additional apps folder to your configuration (config/config.php):
  'apps_paths' =>
  array (
    0 =>
    array (
      'path' => OC::$SERVERROOT.'/apps',
      'url' => '/apps',
      'writable' => false,
    ),
    1 =>
    array (
      'path' => OC::$SERVERROOT.'/apps2',
      'url' => '/apps2',
      'writable' => false,
    )
    ),

3. Done ;-)