hi, try this one this will work
<div>
<asp:ScriptManager runat="server" ID="ScriptManager1"></asp:ScriptManager>
<cc1:ModalPopupExtender ID="MPE" runat="server" CancelControlID="btnCancel"
TargetControlID="HiddenField1" PopupControlID="MessageDiv" BackgroundCssClass="popupbg">
</cc1:ModalPopupExtender>
<asp:HiddenField ID="HiddenField1" runat="server" />
<div runat="server" id="MessageDiv" style="width: 450px;display: none;">
<asp:Label ID="lblMsg" runat="server"></asp:Label><br />
<asp:Button ID="btnOK" runat="server" Text="Ok" OnClick="btnOK_Click" />
<asp:Button ID="btnCancel" runat="server" Text="Cancel" />
</div>
</div>
on code behind ie in your button click
lblMsg.Text = "You will Never shutdown the real napster";
MPE.Show();