VS 2010/VS 2012 Web performance testing & Load testing Errors: Proxy Authorization Required Description: Authorization is required for access to this proxy

To solve this problem we have to add a webtest plug in, the code would be available in the below link.
adding this plugin solved my problem.

http://social.msdn.microsoft.com/Forums/en-US/vstswebtest/thread/651305bf-e30d-4afb-b1d2-78145144478e