Events+ with PayPal showing not paid and IPN problems

Hi,
I'm having some problems I've seen in other threads, but without a conclusion. When people sign up for an event and pay with PayPal, it still shows them as having not paid.

My client tells me that they get notification emails from PayPal requesting to make sure the server handles PayPal IPN

I've configured Events+ so that the client's PayPal email address is set up, but I have a feeling that there may be some settings required on the PayPal side that I haven't done - nor did I find anything in the Events+ documentation about requiring any PayPal settings.

Could somebody please confirm if there's anything that needs to be setup up PayPal (eg a return URL)?
This may help solve my problem, and perhaps these other threads too:

https://premium.wpmudev.org/forums/topic/paypal-payments-on-events-not-showing-paid

https://premium.wpmudev.org/forums/topic/paypal-warning-email-ipn-failing