It seems you want to open your model pop on link button click.
Because you are using Update panel so:-
Wrap your all codes(controls) (model pop up extender + your panel + your linkbutton) inside update panel.
<asp:updatepanel id="updGrid" runat="server">
because you have specified, your TargetControlID is your link button so you don't need to write your click event. Infact it will not fire the click event. if you want to fire click event of your link button then for TargetControlID use some other controls.
hope you got it.