There is an overpass walkway that is technically for the Metro North, but can be used by anyone to get over the tracks. There is an opening in the gate that's just a couple of feet from the staircase up the overpass. Taking the incredibly detailed and totally not made in MS Paint map I created below;
View attachment 7069
Green Box = Parking Lot
Blue/Purple Circle = Gate to exit/enter lot via foot
Black Arrow = direction to go to get to the overpass staircase / elevator
Pink Arrow = When taking I-87 South, get off at Exit 6 "East 153rd Street / River Avenue" and follow the road. Should be the first light where you make a left.
Red Arrow = Upon exiting the lot, make an immediate right, and you will see signs for I-87 N, which will take you to a service road that merges with I-87N.
Due to the multiple levels of roads converging, a 2D map doesn't give the most detailed description of how to get there. When you are approaching the lot, you will pass the supporter's lot on the right. Keep going past a few stop signs, and as I mentioned above, at the first light you will make a left into the lot. Should see a parking dude holding tickets for you to put in your car.
Remember, it is cash only.