If you are adding textareas dynamically and wish to enable tinyMCE on them, you just have to add this on the javascript function that adds said textarea.

tinyMCE.execCommand('mceAddControl',false, id_of_addedtextarea);

Where ‘id_of_addedtextarea’ is what its name implies ;)

More detailed and better info here: http://blog.dileno.com/archive/201102/adding-tinymce-editor-dynamically-and-get-its-value/