Paypal setting

Hi,
I have set up the sandbox for Paypal and it seems to work ok, but it does not tell Admin that the order has been paid. One setting on Paypal was Url for IPN, I assume I should put an url in here but do not know what the url would be, can anyone giver me some idea.
Also there was anothe box ¨Payment Data Transfer¨ what should go here.
Thanks in advance.
Lashman

sandbox is flaky is current version. Live mode works ok tho.
PDT will be supported in the next version

Ok, Thank you Q