Modal Popup Extender and show/hide

<cc1:modalpopupextender id="ModalPopupExtender1" runat="server" BehaviorID="MsgPopup" targetcontrolid="LinkButton1" BackgroundCssClass="modalBackground" PopupControlID="Panel2"></cc1:modalpopupextender>

instead id, use BehaviorId

$find('MsgPopup').show() // to show the ModalPopupExtender.

$find('MsgPopup').hide() // to hide the ModalPopupExtender.