CRM plugin code upgrade

I have converted CRM 4.0 plugin code to 2011, When I try to register new plugin dll, It give me the following error.

Any idea?

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Unable to load plug-in assembly.
Detail: <OrganizationServiceFault xmlns="http://schemas.microsoft.com/xrm/2011/Contracts" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <ErrorCode>-2147204719</ErrorCode>
  <ErrorDetails xmlns:a="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>Unable to load plug-in assembly.</Message>
  <Timestamp>2011-06-01T15:12:05.4973123Z</Timestamp>
  <InnerFault>
    <ErrorCode>-2147204719</ErrorCode>
    <ErrorDetails xmlns:a="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
    <Message>Unable to load plug-in assembly.</Message>
    <Timestamp>2011-06-01T15:12:05.4973123Z</Timestamp>
    <InnerFault i:nil="true" />
    <TraceText i:nil="true" />
  </InnerFault>
  <TraceText i:nil="true" />
</OrganizationServiceFault>

Server stack trace:
   at System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]:
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Microsoft.Xrm.Sdk.IOrganizationService.Create(Entity entity)
   at Microsoft.Xrm.Sdk.Client.OrganizationServiceProxy.CreateCore(Entity entity)
   at Microsoft.Xrm.Sdk.Client.OrganizationServiceProxy.Create(Entity entity)
   at PluginRegistrationTool.RegistrationHelper.RegisterAssembly(CrmOrganization org, String pathToAssembly, CrmPluginAssembly assembly) in C:\Users\remotedev\Desktop\sdk\tools\pluginregistration\RegistrationHelper.cs:line 383
   at PluginRegistrationTool.PluginRegistrationForm.btnRegister_Click(Object sender, EventArgs e) in C:\Users\remotedev\Desktop\sdk\tools\pluginregistration\PluginRegistrationForm.cs:line 447