metabox appearance when category selected

Hi,
I have created a metabox that I want it to appear only when a certain category is selected. The only available solution I found on Google was that on

http://www.farinspace.com/show-hide-meta-box-by-category/

The problem is that when I try the code mentioned in this page, I get the error:
Uncaught ReferenceError: jQuery is not defined.

I actually use jQuery within the metabox without any problem (it is loaded properly with wp_register_script command).

Can anyone suggest any possible solution?
Thank you very much