Arduino Dieciimila ATmega168 trouble connecting

Im working with an [u]Arduino diecimila ATmega 168[/u] and for some reason that I dont know, i cannot upload a program onto it from my iMac. Im running the Arduino 0017 program, im using a simple and completely valid code for a single led, and I still cant get it to upload. I have it on the correct board setting and i've tried all the serial ports and it still wont upload. It must be some problem with the Imac program because the arduino worked fine on the windows computer at school v0017. Somebody PLEASE help me out. Any suggestions will help.