<%@ Control Language="C#" AutoEventWireup="true" CodeFile="_Catalog.ascx.cs" Inherits="_Catalog" %>
<asp:TreeView ID="Tree1" runat="server" BorderStyle="None" EnableTheming="True" OnTreeNodePopulate = "PopulateNode"
    NodeWrap="True" ShowLines="True" Width="250px">
</asp:TreeView>

...
<uc1:_Catalog ID="_Catalog1" runat="server" />
...

TreeNode tmpNode = new TreeNode();
tmpNode.Text = "TEST";
tmpNode.Value = "TEST";        
FindControl("Tree1").Nodes.Add(tmpNode);