paypal single in sandbox mode isn't working (Membership plugin)

It looks like Paypal didn't call back to the site. The new member I created shows as inactive. In paypal (developer area) I do see the purchase confirmation email that was generated for the transaction.

This is installed in multisite (which I certainly wouldn't suspect an issue!)
Is there an IPN activity log I can view in paypal?
What is the IPN url?

Hopefully there's a couple of known hoops to jump through out there