your paypal account would need to be set up to post to an IPN page.
on the IPN page, go to the bindings panel and click the plus button, select eCart -> Checkout -> Checkout Form response
from the gateway list, select paypal IPN Subscription. This will give a bindings group that can be used to get info from the IPN post.
on the IPN Page, you would then have to set up an update record to update the users account based on success or failure of the transaction.
if you need help configuring this, we can help in a premiere support appointment.