get annoying error: form is not defined with AJAX, JQUERY MYSQL Fetch

Question

Hi I've been building a site with a ajax feature that retrievs data from a Mysql database given a surtain variabel.

Here is an example of how it's been done.

http://www.lensfinder.se/testar5.php

I'm given this error in Firefox when I choose different data to retrieve, the AJAX is activated on the select change.

"form is not defined [Break on this error] $.post"

I need some guidance, I've searched around and found that it might have to do with the jquery framework, but I have included it on the site?

Thanks in advance!