Why is it not possible to edit Combobox in ajax control toolkit as in this example

Here is a simple example from Ajax Control Toolkit. I can't edit anything in this control.
What is it that I have missed ?

<asp:ComboBox ID="ComboBox1"  AutoCompleteMode="Append"  DropDownStyle="Simple" runat="server">
        <asp:ListItem>test1</asp:ListItem>
        <asp:ListItem>test2</asp:ListItem>
        <asp:ListItem>test3</asp:ListItem>
    </asp:ComboBox>

//Tony