MeshCollider.sharedMesh  

// Assigns an arbitrary mesh collider to the current transform

var meshToCollide : Mesh;

if(!meshToCollide) {
    Debug.LogError("Assign a mesh in the inspector");
    return;
}
transform.gameObject.AddComponent(MeshCollider);
transform.GetComponent(MeshCollider).sharedMesh = meshToCollide;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Mesh meshToCollide;
    void Example() {
        if (!meshToCollide) {
            Debug.LogError("Assign a mesh in the inspector");
            return;
        }
        transform.gameObject.AddComponent<MeshCollider>();
        transform.GetComponent<MeshCollider>().sharedMesh = meshToCollide;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public meshToCollide as Mesh

    def Example():
        if not meshToCollide:
            Debug.LogError('Assign a mesh in the inspector')
            return
        transform.gameObject.AddComponent[of MeshCollider]()
        transform.GetComponent[of MeshCollider]().sharedMesh = meshToCollide