No popup when clicking on subscription button

On the page: http://speciaalgenot.nl/lid-worden/
I included the subscription buttons. But after clicking on the buttons there is no fancy popup, but an ugly registration form. After filling in the form and trying to register the Ajax won't work and a page appears only saying "0".

I would like it to have the nice popup registration form, but can't seem to get it to work. Any ideas?