Problem with Mesh Collider Collisions

Hey there, i am new in unity and i have a problem.

i modeled my level in cinema 4d and added a mesh collider on it, so that my character can walk on it. it works nicely. i also added the ability to shoot laser to my character, this also works nice,

so when i add a cube in unity and use this script:

 function OnCollisionStay(col : Collision){
    print (col.gameObject.tag);
     if (col.gameObject.tag == "laser"){
     Destroy(gameObject);
     Destroy(col.gameObject);
     }
     }

the cube gets destroyed and the laser"bullet".

but when i try this one on my mesh collider:

 function OnCollisionStay(col : Collision){
     if (col.gameObject.tag=="laser") {
     Destroy(col.gameObject);
 }
     }

it doesnt work. the laser bullet just "flys" through the collider. i dont understand why. i move the bullet with AddForce (just for information).

hopefully you can help, and sorry for my bad english (i am from austria)