OK. I had to poke around the website to make sure I understood where it's located. It's in JJ Byrne Park, in Park Slope, Brooklyn. That's on 5th Ave between 3rd and 4th Sts.
Parking sux in that neighborhood, so leave your car home, if you were thinking about it. Subway: R to either Union St. or 9th St/4th Ave, or F to 4th Ave/9th St.
Or ride your bike, of course.
http://maps.google.com/maps?client=o...HpHOywTxs5nWCQ