Type or namespace membership user cannot be found

- I started an empty ASP.NET 3.5 website
- I included the assembly reference <add assembly="System.Web.ApplicationServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
- I included using System.Web.Security;
- I want to use the following code to add a confirmation code to an email

protected void CreateUserWizard1_SendingMail(object sender, MailMessageEventArgs e)
    {
        MembershipUser user = Membership.GetUser(CreateUserWizard1.UserName);
        string code = user.ProviderUserKey.ToString();
        e.Message.Body = e.Message.Body.Replace("<%ConfirmationCode%>", code);
    }
- I know some changes were made to the location of MembershipUser according to this --> Go to the complete details ...