This route is the easiest and least fun way to access the quarry, but it will get you there. Park in the end of the neighborhood on Waynescott Road and enter the woods to the left of the house with the American flag. There should be a trail to follow that will take you right to the water. You'll have to pass a "No Trespassing - Private Property" sign, but that's how you'll know you're going the right way.

Park at the Ridgewood Baptist church at 2720 Lilly Road and enter the woods near the "No Trespassing - Private Property" signs. If you weave through the woods, you should find a trail that will take you to the upper ridge of the quarry. You can walk the entire distance around the lake (1 mile) on this path and there are a few treacherous routes from the rim down to the water if you want to go in. 

The coordinates of the actual site are 43.137675, -88.206257, which you can plug into Google maps.
Park at NAPA Auto Parts or Sussex Inn in Sussex and walk down the Bugline trail (otherwise known as the sidewalk on Waukesha Avenue). You'll cross train tracks to get there, but there should be a narrow trail.