Hi,
try to use this in plugin:
ClientCredentials credentials = new ClientCredentials();
credentials.Windows.ClientCredential = new System.Net.NetworkCredential("USER", "Password", "Domain");
Uri uri = new Uri("http://server/Organization/XRMServices/2011/Organization.svc");
OrganizationServiceProxy proxy = new OrganizationServiceProxy(uri, null, credentials, null);
proxy.ServiceConfiguration.CurrentServiceEndpoint.Behaviors.Add(new ProxyTypesBehavior());
IOrganizationService service = (IOrganizationService)proxy;