The Object Placer script is a very simple one function script that actually handles the placing of the object down on the Grid. It marks the cell as now full and creates an instance of the preview object from the Grid Selector.
It also has the function of attaching the Grid Object script to any objects being placed. This simple script just stores a reference to the grid it is placed on. This is used by the removal script to clear that cell on that particular grid.
Simply create an empty game object and add ObjectPlacer.cs to it. All you need to do after this is attach it to your scenes grid selector.