It's pretty easy to find - It's right next to Dinousaur BBQ. On the side of Dinosaur opposite the bridge, you'll see a chained off area. If you walk along the fence perpindicular to the road Dinosaur is on, eventually you'll come to an easily accessible opening, which goes directly into the subway. Amusingly enough, the only part of it that's labelled "No tresspassing" is a boring patch of grass you don't need to get to anyways...
You'll probably need:
- Standard hiking supplies (water, food, etc.)
- A good flashlight
- A backup flashlight
- A couple friends to go along with you - just because we didn't run into any disgruntled members of the homeless community, or fall into open sewers, doesn't mean you won't.
You'll also probably want a camera with you.
You have to be careful where you're going. There are long periods of complete darkness with some nasty holes. This place is NOT for the edgy! As long as you scan ahead of yourself with a flashlight and use sound judgement, however, you shouldn't have any real problems.
I went last year with a medium-sized group. We only saw one other person the entire time, emerging from the tunnels right as we went in. Some of the things you'll see: a huge amount of spectacular graffiti, some nice hidden bodies of water, lots of weird objects that have found their way down there, and a close view of the trains going by.
And you can hit up Dinosaur afterwards... exploration is hungry work.
Photos here (starting at the bottom of the page).