$("#signupform")
    .validate({
        rules: {
            password: "required",
            postcode: {
                required: true,
                minlength: 3
            }
        },
        messages: {
            password: "Field Password is required",
            postcode: {
                required: "Field PostCode is required",
                minlength: "Field PostCode must contain at least 3 characters" 
            }
    });