ProSite - PayPal configuration

i'm new in the forum. I've a problem with the ProSite plugin and the PayPal configuration. I hope you can help me to figure out what's wrong :slight_smile:
In short, i've a Wordpress multisite installation, i've installed the ProSite plugin and i've configured PayPal using the "Test mode (sandbox)", the credentials of a PayPal business test account (API Username, API Password, Signature) but i can't enable the sites. I've used a PayPal Personal test account to perform "Pro Upgrade". The payment is successful, but when i'm redirect to the checkout page i see a message like this:

You must pay to enable your site.
XXX €10.00 EUR each month
Payment Method: Your PayPal Account
Last Payment Date: None yet with this subscription (only initial separate single payment has been made, or you've recently modified your subscription)
Next Payment Date: 17/08/2013

and when i go to the site i see the message: "This site is temporarily disabled until payment is received. Please check back later.".

I don't understand what's wrong in the payment process.
I've tried also to specify IPN url, but nothing has changed..