None of the previous answers worked for me. I called the postback of the button on the OnOkScript event.
<div>
<cc1:ModalPopupExtender PopupControlID="Panel1"
ID="ModalPopupExtender1"
runat="server" TargetControlID="LinkButton1" OkControlID="Ok"
OnOkScript="__doPostBack('Ok','')">
</cc1:ModalPopupExtender>
<asp:LinkButton ID="LinkButton1" runat="server">LinkButton</asp:LinkButton>
</div>
<asp:Panel ID="Panel1" runat="server">
<asp:Button ID="Ok" runat="server" Text="Ok" onclick="Ok_Click" />
</asp:Panel>