Cannot complete purchases on subcriptions

Hi,
I have installed the Membership plugin to my site, it all seems to be working well apart from When I try to sign up to a product, the pop-up box appears with my payment options, I click on subscribe and nothing happens (it takes me to a page that shows the same details that are in the pop-up box).

I have tried 2 payment gateway options, Paypal (in sandbox mode) and Free Transaction. Neither of them work.

Furthermore, if I select that registration should be a page not a pop-up in Membership options, when I click subscribe, it just takes me back to the home page. This is a pretty serious problem - can you please take a look?

website is Here - Obviously click on register on the menu to see the issue
Thanks,
Sam

ORIGINAL POST EDITED TO REMOVE USERS ADMIN LOGIN