Take the R3 or R7 train from Trenton and then transfer to the Broad St. line at Suburban or Market East Station. You could transfer to the Broad St. line (orange subway) at Fern Rock if you are taking the R3 (West Trenton) line from Trenton. Fares will depend on the time of travel. Check out the Septa link.
Take the R7 from Trenton to Suburban Station in Center City. There get on a southbound Broad St. Subway and take it to the last stop, which is Pattison Ave. Then follow the crowds of people to the Wachovia Center which you ll be able to see when you come up out of the subway.