Default Mesh colliders are not working

Same Problem here

Set the mesh collider to Convex. There are issues with culled backfaces of mesh colliders not working, among other problems like just being expensive. Consider multiple simple colliders : http://docs.unity3d.com/Documentation/Components/class-SphereCollider.html

If I use custom or compound colliders, Unity does not allow to attach same type of two colliders and if I attach different collider to different child game objects and attach a rigidbody to parent game object in this case It throws Error "Script is trying to access collider blah. blah blah. " . Any Suggestion

Try as alucadj says. Else you can use unity default colliders such as capsule collider for trees or select parts of model (child parts not main parent object) together and apply colliders. Unity automatically sizes them

@PushpaK I have done making collider childs but in the above comment you talked about "apply colliders" Is there any option in unity 3d to apply colliders? So that unity can automatically sizes them as you said