Я подозреваю, что нужно править /plugins/editors/tinymce/jscripts/tiny_mce/tiny_mce.js.
this._def("valid_elements", "+a[id|style|rel|rev|charset|hreflang|dir|lang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur|onclick|ondblclick|onmousedown|onmouseup|onmouseover|onmousemove|onmouseout|onkeypress|onkeydown|onkeyup],-strong/-b[class|style],-em/-i[class|style],-strike[class|style],-u[class|style],#p[id|style|dir|class|align],-ol[class|style],-ul[class|style],-li[class|style],br,img[id|dir|lang|longdesc|usemap|style|class|src|onmouseover|onmouseout|border|alt=|title|hspace|vspace|width|height|align],-sub[style|class],-sup[style|class],-blockquote[dir|style],-table[border=0|cellspacing|cellpadding|width|height|class|align|summary|style|dir|id|lang|bgcolor|background|bordercolor],-tr[id|lang|dir|class|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor],tbody[id|class],thead[id|class],tfoot[id|class],#td[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor|scope],-th[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|scope],caption[id|lang|dir|class|style],-div[id|dir|class|align|style],-span[style|class|align],-pre[class|align|style],address[class|align|style],-h1[id|style|dir|class|align],-h2[id|style|dir|class|align],-h3[id|style|dir|class|align],-h4[id|style|dir|class|align],-h5[id|style|dir|class|align],-h6[id|style|dir|class|align],hr[class|style],-font[face|size|style|id|class|dir|color],dd[id|class|title|style|dir|lang],dl[id|class|title|style|dir|lang],dt[id|class|title|style|dir|lang],cite[title|id|class|style|dir|lang],abbr[title|id|class|style|dir|lang],acronym[title|id|class|style|dir|lang],del[title|id|class|style|dir|lang|datetime|cite],ins[title|id|class|style|dir|lang|datetime|cite]");
Вот в них, видимо, и нужно включить новый тег.