Take NJ transit from NY Penn Station to Trenton Transit Center ($15)
Take a taxi to the route 607 bus stop at South Broad St and Market Street 1/2 mile
($5 + tip) .
Take 607 bus to Bear Tavern Rd at Cardinal Drive ($1.50)
Walk 1/2 mile to Terminal

Both Bus and Train are NJ Transit (NJTRANSIT.COM)
on the Route 607 PDF file, Bear Tavern and Cardinal are approximately 25 minutes from South Broad and Market.

You could save $8 on the taxi by walking but I wouldn't I don't think there are sidewalks and Trenton is not the safest city. Most people use TTN drive to it by car or a small percentage taxi to the West Trenton Train Station to go to Philly. Not easy getting to NYC.