$("body#tinymce").html("mytext");

var id = "yourTextAreaId";
var tmce_el = tinyMCE.get(id);
if (tmce_el != undefined)
{
    tmce_el.execCommand('mceSetContent', false, value);
}
else
{
    document.getElementById(id).innerHTML = value;
    document.getElementById(id).value = value;
}