Can I rez PAD Campaign before my turn starts?
If you look at the revised timing diagram from the Netrunner FAQ, it shows a chance to rez cards just before the corp's "Start of turn effects trigger". So yes, you can rez these assets just before the start of your turn and take advantage of the benefit immediately.

You can rez the PAD Campaign after the runner's last action, just before they discard cards, or just before you draw a card, and still gain the credit. It seems best to rez on your turn so if they have to discard they do so not knowing that your asset was a PAD Campaign.