There is a dirty way to do this. Go to the Menue Manager of your Menue. Choose New->Link-URL. Insert a link name. Insert in the link field of the menu manager: http:///?TB_iframe=true&height=600&width=850" class="thickbox" title=" Omit the leading and the ending double quotes because joomla will add them. Note that joomla will add the menue level class like "mainlevel" or so to your link. This may produce different results in every browser. If you want the link to look like the rest of your menue, you may have to edit the thickbox.css. Note that everytime you edit your link, Joomla will strip the thickbox-class and the title attributes! So you will have to add them (including the quotation marks) again.