I have created a cs file in App_Code

using System;
using Umbraco.Core;
using Umbraco.Core.Persistence;




namespace utData{

    public class MyDB{
        public string demo(){
             var dataContext = new PetaPoco.Database("umbracoDbDSN");
            return "demo - OK";
        }
    }
}

in my web service

 public String dbTest()
   {
       string T;
       T = "Latest testing";
       MyDB d = new MyDB();
       T = d.demo();


   return T;
   }

this crashes

CS0246: The type or namespace name 'PetaPoco' could not be found (are you missing a using directive or an assembly reference?)

var dataContext = new PetaPoco.Database("umbracoDbDSN");

What am I missing in utData?