Trying to follow the guide for extending the Umbraco Forms

But it doesn't seem that the "using Umbraco.Forms.Core.Data.Storage;" is being found as IFormStorage reference cannot be found.

using System;
using System.Web;
using System.Linq;
using System.Collections.Generic;
using Umbraco.Forms.Core;
using Umbraco.Forms.Core.Models;
using Umbraco.Forms.Core.Data.Storage;
using Umbraco.Forms.Core.Enums;


public override IEnumerable<string> ValidateField(Form form, Field field, IEnumerable<object> postedValues, HttpContextBase context, IFormStorage formStorage)
    {

Don't really know where to look, as I've now pretty much just tried to replicate the example in the documentation.