<cc1:modalpopupextender id="modalpopupadd" runat="server" popupcontrolid="pnl_updatePanel" targetcontrolid="btnadd" cancelcontrolid="btn_close">
</cc1:modalpopupextender>
Here Set PopupControlId="Which Panel you Want to Popup",
TargateControlIs="Set Button Id Which is Clicked And Show Popup",
CancleContolId="Set Button Id Which is Clicked And Close Popup"
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/ModalPopup/ModalPopup.aspx[
^]