Problem with user control and $find()

Hello,
i have a usercontrol in one page. When some action is completed successfully then i use one function in which i create panel and modalpopupextender dynamically and add it to one div tag and then scriptmanager.registerstartupscript to do some javascript action . But i am not able to obtain it's reference in javascript. It always returns null. I have tried all possible id's :d like ID, uniqueID,clientID etc but still it always returns null.This is the code. I've even tried to use <%%> but still no luck. If i use Page.Controls.Add() then it always says you should add modalpopupextender between form tag. Don't know what all problems more it will give. :(
private void AddPanelAndShowModalPopup()
  {
  Panel objPanel = new Panel();
  objPanel.ID = "pnlPhotoPopUp";
  objPanel.CssClass = "cssInvisible"; //objPanel.Attributes.Add("display","none") doesn't work for some ...