jQuery(document).ready(function () {

    jQuery("#registrarse").validate({

        errorClass: "msg_contacto",
        onkeyup: false,
        rules: {
            im_Nombre: {
                required: true,
                minlength: 3
            },
            im_Email: {
                required: true,
                email: true
            },
            im_cEmail: {
                required: true,
                email: true,
                equalTo: "#registrarse #im_Email"
            },
            im_Telefono: {
                required: true
            }
        },
        messages: {
            im_Nombre: {
                required: 'Campo Requerido',
                minlength: 'Su nombre debe tener al menos 3 caracteres'
            },
            im_Email: {
                required: 'Campo Requerido',
                email: 'E-mail inválido',
            },
            im_cEmail: {
                required: 'Campo Requerido',
                email: 'E-mail inválido',
                equalTo: 'Confirme correctamente su E-mail'
            },
            im_Telefono: {
                required: 'Campo Requerido'
            }
        },
        errorPlacement: function (error, element) {

            error.appendTo(element.nextAll("span.mensaje"));
        }

    });

});

  Here is the link to jsfiddle: