The Port Authority bus should start running 2.5 hours before the show - so around 4:30 - and will continue until 7:30.
Here's the NJ Transit page about it with some good info:
http://www.njtransit.com/nn_mbs_4.shtml
My advice if you're taking the bus: get there EARLY. The drive from the Port Authority to the Meadowlands should take less then 30 minutes, but if traffic is bad it can take over an hour. The bus is first come first served so the later you get there and get on a bus the more likely you are to be late for the show. Also, after the show you'll want to get back to the bus stop as soon as possible because they don't run buses all night and you don't want to be left there in the middle of the swamp with no way to get back!
(This is why I am renting a car. LOL)