Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %><asp:Content ID="PageHead" ContentPlaceHolderID="PlaceHolderAdditionalPageHead" runat="server"><link href="../Stylesheet.css" rel="stylesheet" type="text/css" /></asp:Content><asp:Content ID="Main" ContentPlaceHolderID="PlaceHolderMain" runat="server"><div id="wrap"><div id="header"><h1>
                Admin</h1></div><div id="nav"><asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" ></asp:ModalPopupExtender><asp:Button ID="Edit" runat="server" Text="Edit" /></div>

Hello, I want to use Ajax Toolkit in an application page of a SharePoint project. But I got this error: Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies.

I used to use it in asp.net project but it was okay. Not sure why?

My code is above, thanks.