Yeap, you were right. That line can't be found on prosilver. I had a look into the files, and I think it's using 'div' to define textarea. But it has an id, 'posting box'. So you might want to try exchange the id to 'posting box' first and see what will happen. Other than that, I really have no idea. Maybe you can try the official forum, phpBB.com - ask them where the textarea for prosilver is.

Regarding double post, I don't have that problem on my chrome and FF. Sad thing is popup won't appear at all on IE8. I've tried on several other computers too. One thing you can do is check on here http://www.codecogs.com/components/equationeditor/equation_install.php#Quick See if you have the same double popups when you click the links in the example.

Cheers