As per my understanding on click if the ok button you want to validate the entire form
if so you can perform client side validation by calling a client side script function in the ModalPopupExtender properties
<ajaxToolkit:ModalPopupExtender ID="MPE" runat="server"
TargetControlID="LinkButton1"
PopupControlID="Panel1"
BackgroundCssClass="modalBackground"
DropShadow="true"
OkControlID="OkButton"
OnOkScript="onOk()"
CancelControlID="CancelButton"
PopupDragHandleControlID="Panel3" />
Where define a OnOk () java script and validate your form
Or else if you want to validate or perform server side operation then inside ur target control use html.beginform ('actionname','controllername')
In order to perform the action on ok of that form.
Hope it helps