error CS0246

I hope someone can shed some light on this I'm new to C# coding so...
I'm trying to have a class that would be used on two separate pages and am not having much luck.
I keep getting this error for both pages: "The type or namespace name 'DPTypes' could not be found (are you missing a using directive or an assembly reference?) " 

One page has this at the top of the page:
 
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

using DPTypes;
 
The other page has this: 
<%@ Page Language="C#" AutoEventWireup="true" %>
<%@ Import Namespace="System" %>
<%@ Import Names ...