mac and serial monitor

hi to all.. i'm new in arduino. i want to write hello world in my serial monitor. i'm using mac osx 10.7 but the serial monitor don't work..

the code is a simple hello world:

void setup()                    
{
 Serial.begin(9600);           


}

void loop()                      
{
         Serial.println("Hello world!");  
          delay(2000);                
}

i don't understand why the serial monitor don't display hello world. thank you.

i don't understand why the serial monitor don't display hello world.

Does it display anything? Do you have the USB cable connected?

yes i have the usb plug in.. the rx led is always on, but the serial monitor is white.. i read some posts in the internet, could be that i don't have installed rxtx on my mac? in windows it works, and i saw that in the folder, there is a file called rxtx..