Details of styling and options can be found on the jQuery Treeview plugin page, but an example of how you might change the menu behaviour might be
<ddr:MENU MenuStyle="Treeview" runat="server">
<ClientOptions>
<ddr:ClientString Name="animated" Value="slow" />
<ddr:ClientBoolean Name="unique" Value="true" />
</ClientOptions>
</ddr:MENU>
which looks like this: