Please use the following code:

 

CuteEditor.ToolControl tc = Editor1.ToolControls["LinkTree"];
   CuteEditor.TreeDropDownList tdd = (CuteEditor.TreeDropDownList) tc.Control;   
   
   //clear the items from configuration files
   //see Configuration/Shared/Common.config
   //tdd.Items.Clear();
     
   //Add items by code
   CuteEditor.TreeListItem rootitem=new CuteEditor.TreeListItem("Root",null);
   rootitem.Selectable=false;
   tdd.Items.Add(rootitem);
   rootitem.Items.Add("Asp<font color=red>.Net</font>","Asp.Net","http://asp.net");
   rootitem.Items.Add("DotNetNuke.Net","DotNetNuke.Net","http://DotNetNuke.com");
   rootitem.Items.Add("CuteSoft","CuteSoft","http://CuteSoft.net");