$('#myForm').validate({ // initialize the plugin
    rules: {
        "UserDetails.SelectedCountry": "required",
        "UserDetails.SelectedState": {
            required: function (element) {
                var value = $("#UserDetails_SelectedState").val() == 0;

                if (value) { // If the value is true I need to return false to make it required
                    alert("here");
                    return false;
                } else { // Otherwise value is greater then 0 ok to carry on,
                    return true;
                }

            }
        }
        "UserDetails.Postcode": "required"
    },
    messages: {
        "UserDetails.SelectedCountry": "Please choose your country",
        "UserDetails.SelectedState": "Please choose your state",
        "UserDetails.Postcode": "Please enter your postcode",
    }
});