Supporter plugin: don't see any change to wp-signup.php

Hi there,
I've just installed the Supporter plugin on my new multi-site installation. I don't need any of the optional pages - I just want people to pay to get an account - so I've installed the minimum set of files as described in install.txt (and the online supporter documentation), plus the two paypal files. The only oddity I saw in this process is that my WP install doesn't have a mu-plugins directory, the directory is just called plugins. I don't believe that's a problem - just a change in newer versions of multi-site.

So... after copying the files over, I've done a network activate on the plugin, and done some configuration in the Admin pages (under Settings / Supporter). From what I've read, I should now be able to go to the page and it should have some of my custom text from the supporter plugin, and people will have to use Paypal to create an account.

However, the page seems to be completely unchanged - as if Supporter wasn't installed.

I'm not sure what I'm missing. Can anyone help?