Problem with arduino on a mac - no usb to serial port driver

I have an atmega328 with a separate programmer. I downloaded the arduino software for my windows comp, and It works find; I can program the board, but on my mac (the most convenient computer for me to use) I always get the "programmer is not responding error. Apparently, I don't have a usb to serial driver, but when I go to download the driver suggested on the arduino "getting started on a mac" page, it still wont show up under tools/serial port. Do I have the right driver? Are there alternate drivers that may work better? Any help would be appreciated.

It's not clear from your post if you have an Arduino or if you are using the Arduino IDE to program an atmega328 with a stand alone programmer.

If you are using the programmer to program the 328, what programmer are you using? That will determine what drivers you may need for the Mac.

If you are using an Arduino, what Arduino are you using?