It sounds like your gravity is set in the opposite direction. Do the default cubes in Unity behave the same way when you add a rigidbody to them?
I have used unity for about a year. When you use a cube, a sphere or any of the other common 3D game objects, there are no problems adding a rigid body.
I suppose you might be right about opposite gravity, because that was my first thought, but I just didn't know how to reverse the gravity again, because I am not an expert.