Here are the steps you can take to use RssDataSource in your page or user control:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>
<%@ Register Assembly="RssToolkit" Namespace="RssToolkit.Web.WebControls" TagPrefix="ast" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Consuming Google News using RssToolkit</title>
</head>
<body>
    Google News (Atom Format)
    <form id="form1" runat="server">
       <ast:RssDataSource id="RssDataSource1" runat="server" maxitems="5" url="http://news.google.com/?output=atom"></ast:RssDataSource>
       <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataSourceID="RssDataSource1">
          <Columns>
             <asp:HyperLinkField DataNavigateUrlFields="link" DataTextField="link" HeaderText="Link"/>
             <asp:BoundField DataField="title" HeaderText="Title" SortExpression="title" />
          </Columns>
       </asp:GridView>
    </form>
</body>
</html>