Thanks a lot for your quick response. I did checked the example, and added input manager into the user control. It turned out it was working perfectly in the example project. But still no luck in my own project, even the implementation are almost the same.
After that, I used Firebug to inspect on the codes behavior, I did noticed that when I leave the textbox with no text filled, the css class of the textbox has been changed, with a css class name contained "ERROR" been added. But I did checked the CSS comes with the pages but couldn't find. If I disable the AJAX function from AJAXManager, I can see the css generated as a resource of page. I think this is the problem. Do you have any idea that why the css resources were not generated with page for input manager?