MVC Model require true

I don't know of a way through DataAnnotations, but this is easily done in your controller.

public ActionResult Add(Domain.Something model)
{

    if (!model.MyCheckBox)
        ModelState.AddModelError("MyCheckBox", "You forgot to click accept");

    if (ModelState.IsValid) {
        //'# do your stuff
    }

}

The only other option would be to build a custom validator for the server side and a remote validator for the client side (remote validation is only available in MVC3+)

Do you have the appropriate items set up in the web.config?

That could cause the validation not to work.

You can also try to create a custom validation attribute (since [Required] only cares whether or not it exists, and you care about the value):

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
sealed public class RequiredTrueAttribute : ValidationAttribute
{
    // Internal field to hold the mask value.
    readonly bool accepted;

    public bool Accepted
    {
        get { return accepted; }
    }

    public RequiredTrueAttribute(bool accepted)
    {
        this.accepted = accepted;
    }

    public override bool IsValid(object value)
    {
        bool isAccepted = (bool)value;
        return (isAccepted == true);
    }

    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentCulture,
          ErrorMessageString, name, this.Accepted);
    }
}

Then, usage:

[RequiredTrue(ErrorMessage="{0} requires acceptance to continue.")]
public bool Agreement {get; set;}

From here.