about add product,add cross sells and add related product button in admin panel

Hello there.........
i have made design changes in the front end of nopcommerce 1.8...
i didn't change anything in admin side....The code as well as the design of admin panel remains as it is as nopcommerce 1.8..
In admin panel whenever i go to category from catalog it would show various categories....
and if i edit in particular category i will be redirected in categories detail page...now by selecting product tag panel i wiil be able to see "Add product " button......
when i click on that button the page gets postback bt the pop up window in which i can add a new product would'nt open....

The code for the button is----------

<asp:Button ID="btnAddNew" runat="server" CssClass="adminButton" Text="<% $NopResources:Admin.RelatedProducts.AddNewButton.Text %>" />

and code is------

btnAddNew.OnClientClick = string.Format("javascript:OpenWindow('RelatedProductAdd.aspx?pid={0}&BtnId={1}', 800, 600, true); return false;", this.ProductId, btnRefresh.ClientID);

please help me by solving this problem as early as possible

thank you.........