Use below code in place of your code ,
<asp:scriptmanager id="ScriptManager1" runat="server">
</asp:scriptmanager>
<asp:ImageButton ID="ImageButton1" runat="server" CssClass="search_img" ImageUrl="images/search.png" ></asp:ImageButton>
<cc1:modalpopupextender id="ModalPopupExtender1" runat="server"
cancelcontrolid="btnCancel" okcontrolid="btnOkay"
targetcontrolid="ImageButton1" popupcontrolid="Panel1"
popupdraghandlecontrolid="PopupHeader" drag="true"
backgroundcssclass="ModalPopupBG">
</cc1:modalpopupextender>
<div id="Panel1" style="display: none;" >
<div class="HellowWorldPopup">
<div class="PopupHeader" id="PopupHeader">Header</div>
<div class="PopupBody">
<p>This is a simple modal dialog</p>
</div>
<div class="Controls">
<input id="btnOkay" type="button" value="Done" />
<input id="btnCancel" type="button" value="Cancel" />
</div>
</div>
</div>
add all CSS classes which are available in your referred link,and use class names exactly.