Are you using your sandbox credentials for the transaction? A sandbox account is completely separate and different than a live account and you must ensure that you have the sandbox credentials in place when you are testing. Also, after you launch the sandbox you will need to complete the transaction there and get back to your checkout success page before you would be able to see any transaction details. One other thing is that when you are testing you should have a sandbox user that is different than your main sandbox account since you are not allowed to make PayPal purchases from yourself.
