Please help with beginner Prefab & OnMouseDown

Hello, I am working on a card game for the web. I have four prefabs for the backs of the cards: blue, red, yellow, green. Right now I can push play and the script instantiates a blue card, but I can't figure out how to register a click on this card.

Attached to the main camera, instantiate_prefabs.js:

var blue_card : GameObject;

function Start () {
 var blue_card   = Instantiate(blue_card,   
                                 Vector3 (0, 0, 0), Quaternion.identity);    
}

Attached to each of the four prefabs for blue_card, red_card, yellow_card, green_card, flip_card.js:

function OnMouseDown() {

        Debug.Log("Prefab instance clicked.");
}

Sorry if this code is weird, I am baffled. Each card is made of a UV Plane, courtesy of a graphic artist, so please go easy as I don't know anything about 3D art. Thank you so much.

Is there a collider attached to the prefabs?