To add to the confusion, there is a second cruise terminal which was used before the new one was built. It is called Xinggang though this name may be used for the new terminal.
Directions to Tianjin Cruise Terminal from Princess with directions in Chinese.
Otherwise, you need to arrange either a tour company or taxi to take you into town. Taxi drivers turn off their meters and want a whopping 300 Yuan to take you to the railway station at Tanggu (it should only cost 80 Yuan on the meter). You'll need to negotiate to get a better rate but taxis are generally very inexpensive in China.
When taking taxis, have your destination written in Chinese since most taxi drivers don't speak English. Taxi drivers from Tianjin City may have problem finding the terminal since it is so far away outside the city.
Bullet trains run regularly from Beijing South Railway Station to Tianjin but trains that continue to Tanggu are less frequent.
Tour company offering transport to terminal http://www.tour-beijing.com/xingang_beijing/
Train Schedules http://www.chinatrainguide.com/