Very simple. After you pass the Pleasantville tolls (at approx the 2 mile marker - its a 50cent toll) you will pass the visitors center, mini-mart, employee parking). Stay in the right lane and you'll be guided into it.

Its not an immediate tunnel, you'll pass the RR tracks and then be put into the tunnel. Watch the signs when you get out. You are looking for the Marina exit. If you go too far you end up over the bridge in Brigintine - no worries though a quick U-turn and your back at the Marina.