Discover the URL for the organization using global discovery service not working.

Hi,

I am referring to msdn.microsoft.com/.../mt607485.aspx  section 'Using the Discovery service'.

Here I am trying to discover my organization using global discovery service but getting below error  

GET globaldisco.crm.dynamics.com/.../Instances(UniqueName='MyOrgUniqueName')
Authorization : Bearer <access_token>
{"error":{"code":"","message":"Authorization has been denied for this request."
  }
}

Am I missing something here?
Could you please suggest how can I retrieve organization URL?

Thanks,