Buzzy,
I'm not sure what's happening, but a core function of Mootools (the Joomla Javascript Library) that we call isn't working on your pages. We use the 'fade' function to show the popup. On your site though, when we call that function, there's an error message that says:
Uncaught TypeError: Object [object Object] has no method 'fade'
However, if I load your page with a different template by adding ?template=atomic to the URL, the popup works as expected. I'm not sure what's causing the problem, but I'd recommend either disabling modules and plugins to see which one is interfering with the Mootools library. Hopefully, you can narrow it down to one other specific extension and then we can help you figure out where the conflict is coming from.
Hope that helps get you started,
Alex