This message means that Wild Apricot did not receive a proper payment confirmation from PayPal system. The registrant has been added to the list in 'Pending' status. What you should do is to check in your PayPal account whether the payment actually went through. If it did - simply change the status from Pending to Confirmed. If not - you can either ask them to send you a check or manually via PayPal- or delete this registration and ask them to try again and pay online.
Dmitry Buterin, Chief Apricot