Well i don't know if it is possible to add the dnn user controls to the vs toolbox. But if you want to add the editor to a page just use
<%@RegisterTagPrefix="dnn"TagName="texteditor"Src="~/controls/texteditor.ascx"%>
<dnn:texteditorid="txtContent"runat="server"height="400"width="100%"></dnn:texteditor>