What's notification URL to inform in PayPal?


I’m configuring www.abem.pro.br/VIIIEBEM payment for Paypal, and I use https://www.sandbox.paypal.com/cgi-bin/webscr to test… and in my test when I click to PAY with Paypal this message appear

“Things don’t appear to be working at the moment. Please try again later.”

What I do wrong?

I solved… but the problem is when the message back… how the OCS will receive it?

I open another ticket for this!