We have removed the parameter "Use on Custom Objects" because this feature now is always enabled. Really, it had no sense to disable it.
To use Random Tint, you must use "Forest Material" or "Forest Color" in your materials. The first one is a full material, used mainly for bilboards, and "Forest Color" is a texture map, which works better with 3D objects.
These would be the basic steps to use the Tint with a ground map:
1) Add Forest Color to some slot (usually to Diffuse) of the custom object's material. If you was using a texture for this slot, assign it to the "Default" parameter.
2) In the Forest object, Material rollout, set the Range (different from 0%), set mode to Map->As Texture, and assign the texture to the Map slot.
In this way, each instance of Forest Color takes the Tint parameters from the Forest object. But you can also adjust them individually, turning on the "Override" checkbox in Forest Color, and introducing the new values.
Forest Color also allows you to use some advanced features, as random textures or tint by element. They are described in the help reference.
I have attached a basic sample scene.