My friend, since you're traveling on NJ Transit into the City, the easiest, fastest and cheapest way to get from Penn Station to the Beacon Theater, which is located on West 74th Street and Broadway, is by subway.
Take the uptown (northbound) number 2 or 3 subway lines, which is the red line on the subway map at the link below, from 34th Street-Penn Station to 72nd Street.
Once you exit the subway at 72nd Street, walk two blocks north along Broadway from the subway station to the theater as indicated on the map at the link below.
The trip by subway from Penn Station to the 72nd Street take around 10 minutes, plus an addition two to three minutes walk from the 72nd Street subway station to the theater.
The one way fare on the subway is $2.25 with a pay per ride (regular fare) MetroCard or $2.50 with SingleRide ticket.
I hope this information is very helpful.
Native New Yorker