Grid Framework works really well. If you're set on trying to do it in playmaker, I've got this to work but taking the Position of the Object, saving it as a float, converting it to an Integer (whole number), then saving it back to that float and setting the position of the object. So if you dropped your object at 1.2, 0, 1.4, it would become 1, 0, 1. But Grid Framework definitely is awesome and would save you the headache.