Open a web page in a content place holder

Dear,
 
I have created a side menu bar in ASP.Net using CSS friendly adapter controls. Now I want this page to open in the content place holder located at the left of my page so that it looks perfect. Currently my content page structure is:
 

<%@ Page Language="C#" MasterPageFile="~/TwoColumn.master" Title="Untitled Page" %>
 
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">Left Stuff
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">Right Stuff
</asp:Content>

 
Kindly help me getting a solution. Thanking you in advance.
 
IB