Include following code at the top of ascx page

<%@ Register TagPrefix="dnn" TagName="TREEVIEW" src="~/admin/Skins/TreeViewMenu.ascx"%>

SkinObject	Code Snippet	Description	Example
dnn:TREEVIEWMENU	< dnn:TreeViewMenu runat="server" id="dnnTreeViewMenu">	Display a navigation Menu using the DNN Treeview Control to provide a Windows Explore like Menu.