Paypal Express not working with Webforms CIVICRM Intergration
This has been a known bug/issue in webform_civicrm ( see e.g. Webform CiviCRM Paypal Button not Working ) -> try upgrading to the latest webform_civicrm (dev -> clone or download master: https://github.com/colemanw/webform_civicrm.git ). 

I also strongly encourage you to upgrade your CiviCRM - there have not been any security releases for 4.5.x for quite some time now.