The type or namespace Compare could not be found (are you missing a using directive or an assembly reference?)

The type or namespace Compare could not be found (are you missing a using
directive or an assembly reference?)

I'm trying to add an Admin to my site. There issue is found on my
AccountsModels.cs
It simply has to compare the data implemented but I seem to get this error.
I also have a view with:
-Register.cshtml
-LogOn.cshtml
-ChangePasswordSuccess.cshtml
-ChangePassword.cshtml
& an AccountController.cs of course..
Someone know a solution?
Here is the code:
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
using System.Web.Security;
namespace Videoteek.Domain.Models
{
public class ChangePasswordModel
{
[Required]
[DataType(DataType.Password)]
[Display(Name = "Current password")]
public string OldPassword { get; set; }
[Required]
[StringLength(100, ErrorMessage = "The {0} must be at least {2}
characters long.", MinimumLength = 6)]
[DataType(DataType.Password)]
[Display(Name = "New password")]
public string NewPassword { get; set; }
[DataType(DataType.Password)]
[Display(Name = "Confirm new password")]
[Compare("NewPassword", ErrorMessage = "The new password and
confirmation password do not match.")]
public string ConfirmPassword { get; set; }
}
public class LogOnModel
{
[Required]
[Display(Name = "User name")]
public string UserName { get; set; }
[Required]
[DataType(DataType.Password)]
[Display(Name = "Password")]
public string Password { get; set; }
[Display(Name = "Remember me?")]
public bool RememberMe { get; set; }
}
public class RegisterModel
{
[Required]
[Display(Name = "User name")]
public string UserName { get; set; }
[Required]
[DataType(DataType.EmailAddress)]
[Display(Name = "Email address")]
public string Email { get; set; }
[Required]
[StringLength(100, ErrorMessage = "The {0} must be at least {2}
characters long.", MinimumLength = 6)]
[DataType(DataType.Password)]
[Display(Name = "Password")]
public string Password { get; set; }
[DataType(DataType.Password)]
[Display(Name = "Confirm password")]
[Compare("Password", ErrorMessage = "The password and confirmation
password do not match.")]
public string ConfirmPassword { get; set; }
//****
[Required]
[Display(Name = "Security Question")]
public string PwdQuestion { get; set; }
[Required]
[Display(Name = "Security Answer")]
public string PwdAnswer { get; set; }
}
}