Just did a fresh install of Buddypress here:
I've tried activating supporter and making it work on the signup page.
While I do see the supporter form on the signup page,
If I choose the "I'm Interested" option, it still won't take me to the paypal payment page.
After signup I can see supporter working just fine on the dashboard.
In addition trying to make it work using the url
(I don't know if it's related):
wp-signup.php - loads blank as well as wp-signup.php?Supporter=1
I've read a similar post on the forum but the problem there was that the free days option was turned on and I have free days set to 0.
I just can't seem to get this to work.
This site is not up so feel free to try and signup and play around if you like.