internal sealed class usersMetadata
        {

           ...
            [CustomValidation(validatorType: typeof(MyValidators), method: "IsLoginAvailable")]
            [Display(Name = "Имя входа", Order = 2)]
            //[Required(ErrorMessageResourceName = "ValidationErrorRequiredField", ErrorMessageResourceType = typeof(ValidationErrorResources))]
            //[RegularExpression("^[а-яА-Яa-zA-Z0-9_]*$", ErrorMessageResourceName = "ValidationErrorInvalidUserName", ErrorMessageResourceType = typeof(ValidationErrorResources))]
            public string login { get; set; }
           ...
     }

namespace MyApp.Web.Models
{
    public class MyValidators
    {
        public static ValidationResult IsLoginAvailable(string value)
        {

            return new ValidationResult(String.Format("Пользователь с именем входа \"{0}\" уже существует ", value));

        }