your controlID for the table needs to be the chkList
<asp:TextBox ID="txtCombo" runat="server" ReadOnly="true" ControlID="chkList" >
<cc1:PopupControlExtender ID="PopupControlExtender1" runat="server"
TargetControlID="txtBox" Position="Bottom"
PopupControlID="panel3" >
the textbox needs to be controlled by the selected value of the checklist
i hope this helps and i have interpreted the question correctly