This solution did not seem to work for me - I have reached a dead end:

I have edited the js file exactly as described above - and copy/pasted straight from A to B.
I have checked for empty spaces and made sure a comma is placed between the iframe and the original code.

I have pasted the iframe code into the HTML box of the WYSIWYG editor. The iframe code is a
form code copy/pasted directly from Google spreadsheets. Same code works in a joomla wrapper.

I have refreshed page and flushed Firefox - still no iframe is showing.

I have tried to attatch a zipped copy of the tinymce.template.js file - in case anybody wanted to take a look - but get this error (i dont know - maybe this is bcause one cannot upload zipfiles into these forums?):

Fatal error: Call to a member function setField() on a non-object in /sites/ssopen/www/forum/code/Forum.php on line 647.

The website and page where the iframe is meant to be is http://www.silverstripe.dk/kontakt-os/

Really hope somebody can help out? Thanks!