Testing payments with PayPal is quite complicated, since they require using their distinct sandbox area. Follow this procedure.

1. Register at developer.paypal.com
2. Create a personal account (test user to pay).
3. Confirm your email via the "email" tab of developer central and add a credit card (use the default credit card info it gives you).
4. Create a business account
5. Enter the correct IPN URL (site.com/wsnlinks/paypal.php) for this business account.
6. At the WSN Links admin panel, Customization->Templates, change the corresponding URL at sponsor.tpl template in order to post to www.sandbox.paypal.com.
7. At the WSN Links admin panel, Settings->Sponsorship, enter the corresponding email for your test business sandbox paypal account.
8. At the top of paypal.php change $sandbox = false; to $sandbox = true;
9. Finally you have to be loged in at developer.paypal.com to test trial payments.
10. If you want some debug info emailed to you, add a $debug = 1; line.