Updating a ListBox via a Text Box

I am currently working with a large query of client name, and instead of using a dropdownlist, I have done the follow: 
I setup a Textbox for the user to enter some text, and then click the search button.  Using a modalpopupextender (AJAX), and panel appears showing a listbox displaying clients that are like the text entered.  The user then picks one, hits ok, and the value is returned to the text box. 
Currently, the listbox is not displaying anything.  Here is what I have:
<asp:TextBox ID="tbClientDirectName" runat="server" /> <asp:ImageButton ID="ImgBtnClient" ImageUrl="images/Search.gif" runat="server" /> <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="ImgBtnClient" PopupControlID="pnlClient" CancelControlID=&quo ...