Rigidbody/mesh collider

Hey i was just wondering I am having a bit of trouble with the mesh collider fir my game.

I imported my zombie asset from store and i want to add the rigidbody and mesh collider so he actually stays grounded but once i attack them turn collider to convex u all he does is fall backward and bounce around wile trying to get to me. i have noticed the actual mesh of the character isnt aligned as the mesh is in the star shape( arms stretched out standing straight as were my char is bent over as though an animation is paused i think this is what is causing the problem. if anyone could tell me best way to go about this would be great thanks :)