Walk in to Penn from 7th ave under the MSG sign. Walk dead straight past the NJT stuff to where the Amtrak waiting room area is (was? haven't been on the train in a while).

Before you get to the oval you have to walk around, there is a Wells Fargo ATM, and next to it an ancient CRT monitor with trains and tracks. Go down the stairs there to the lower level once you find your track, and there you can take the middle corridor (Hilton Corridor?) to the stairs nearer you than Moynihan.

See this map. It's the third map as you walk into the station, then the 4th one when you get downstairs. Handicapped elevators are a good trick, too.