WordPress and jQuery

Hi all,

I try to integrate the following javascript code in my plugin

[Code moderated as per the Forum Rules. The maximum number of lines of code that you can post in these forums is ten lines. Please use the pastebin]

for displaying a metabox if a certain category is selected. But the problem is that the following error occurs: jQuery is not defined.
This is strange since I load correctly jQuery with wp_register_script and I use it elsewhere in my plugin.

Can you please help me with this?
Thank you very much!!