Can not get WCF service running with Nhibernate

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C#

C#4.0

Hi Masters!
I try to understand my problem but finally after googling many times, i couldn't find out the solution.
My problem is:
I create a project using WCF service library template.
Add reference to Nhibernate dll.
Add using Nhibernate to the code
and try to add the following code
NHibernate.Cfg.Configuration config = new NHibernate.Cfg.Configuration();
ISessionFactory sessionFactory = config.BuildSessionFactory();
ISession session = sessionFactory.OpenSession();

Finally i build it with Error
The type or namespace name 'NHibernate' could not be found (are you missing a using directive or an assembly reference?)
Below is the code of my service class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.Text;
using NHibernate;
namespace WcfServiceLibrary1
{
        // NOTE: You can use the "Rename" command on the "Refactor" menu to change the class name "Service1" in both code and config file together.
        public class Service1 : IService1
        {
                public string GetData(int value)
                {
                        return string.Format("You entered: {0}", value);
                }
                public CompositeType GetDataUsingDataContract(CompositeType composite)
                {
                        if (composite == null)
                        {
                                throw new ArgumentNullException("composite");
                        }
                        if (composite.BoolValue)
                        {
                                composite.StringValue += "Suffix";
                        }
                        return composite;
                }

                public void GetDataFromNhibernate()
                {
                        NHibernate.Cfg.Configuration config = new NHibernate.Cfg.Configuration();
                        ISessionFactory sessionFactory = config.BuildSessionFactory();
                        ISession session = sessionFactory.OpenSession();
                }
        }
}

I don't know why. Can anyone help me?
THanks