I am not able to register custom work flow

Hi all,

I am using this code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Workflow;
using System.Activities;
using Microsoft.Xrm.Sdk.Query;
using Microsoft.Xrm.Sdk.Messages;//RetrieveRequest naemspace
using Microsoft.Crm.Sdk.Messages;
using System.ServiceModel.Description;

namespace Test
{
public class RoundRobinLeadAssignment : CodeActivity
{
protected override void Execute(CodeActivityContext context)
{
ITracingService tracingService = context.GetExtension<ITracingService>();
IWorkflowContext workflowContext = context.GetExtension<IWorkflowContext>();
IOrganizationServiceFactory serviceFactory = context.GetExtension<IOrganizationServiceFactory>();
IOrganizationService service = serviceFactory.CreateOrganizationService(workflowContext.UserId);

//My cystom logic here
}
}
}

Reference below sdk asseblies:

Microsoft.Crm.Sdk.proxy

Microsoft.Xrm.Sdk

Microsoft.Xrm.Sdk.Workflow

use IL merging for reference assemblies.

When I try to register my assembly I have the below error:

I tried the link which is provided by  Guido Preite for SDK 6.1.1 But this is not available for me and showed following error

What is wrong? 

Thank You