Scripting Reference
Input.GetMouseButtonDown

static function GetMouseButtonDown (button : int) : boolean

Description

Returns true during the frame the user pressed the given mouse button.

You need to call this function from the Update function, since the state gets reset each frame. It will not return true until the user has released the mouse button and pressed it again. button values are 0 for left button, 1 for right button, 2 for the middle button.

JavaScript 
// Detects clicks from the mouse and prints a message
// depending on the click detected.

function Update() {
    if(Input.GetMouseButtonDown(0))
        Debug.Log("Pressed left click.");
    if(Input.GetMouseButtonDown(1))
        Debug.Log("Pressed right click.");
    if(Input.GetMouseButtonDown(2))
        Debug.Log("Pressed middle click.");
}
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Update() {
        if (Input.GetMouseButtonDown(0))
            Debug.Log("Pressed left click.");
        
        if (Input.GetMouseButtonDown(1))
            Debug.Log("Pressed right click.");
        
        if (Input.GetMouseButtonDown(2))
            Debug.Log("Pressed middle click.");
        
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Update():
        if Input.GetMouseButtonDown(0):
            Debug.Log('Pressed left click.')
        if Input.GetMouseButtonDown(1):
            Debug.Log('Pressed right click.')
        if Input.GetMouseButtonDown(2):
            Debug.Log('Pressed middle click.')