Follow the steps.
Go to RowDataBound event of your GridView --> Find the GridView footer --> Find your target control --> Set it as TargetControl of your ModelPopupExtender.
Try this:
protected void GridViewName_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.Footer)
{
Label lbladd = (Label)e.Row.FindControl("lbladd");
modelPopupExtender1.TargetControlID = lblad.ClientID;
}
}
<asp:modalpopupextender runat="server" id="modelPopupExtender1" targetcontrolid="tempLink" popupcontrolid="popUpPanel" backgroundcssclass="modalBackground" xmlns:asp="#unknown">
<asp:linkbutton id="tempLink" runat="server" visible="false" />
</asp:modalpopupextender>
--Amit