If you must go to Port Authority bus stop from NYC Penn station, my
advise would be to board towards the front of the LIRR train from Jamaica..
at Penn Station get out towards 8th avenue. Catch a cab or walk on
the 8th avenue. Its about 8-10 minutes walk.
http://maps.google.com/maps?f=d&sadd...,0.012381&z=16
Besides Amtrak from Penn station, Bolt bus is also an option.
http://www.boltbus.com/ - no personal experience with them yet.