How to add gravity to imported objects?

I imported a model into unity and got it to move using transform. I tried to add gravity so that it falls onto a platform and once its off it falls (3D). i added Rigid Body but if falls right through the platform (cube). IDK what to do and how to get it to stay on the platform.

PS The object is made up of multiple objects.

hi;

the model and the platform should have colliders;
to stay on the platform u need to add a rigidbody to the platform too;
but the platform rigid body the graivity option should be unchecked and the IsKinematic option should be checked;
this way it wont fall and wont let the model goes throght;