You set the baud rate, etc in your Mac (or Windows) terminal program, and also in the Arduino software you write. They have to match. I think the Arduino serial library uses 9600 8N1 as the default. The Arduino IDE has a very simple built-in serial terminal that you can use to see debug text coming back from the board. There are also a couple of serial port example programs that come with the IDE. On the Mac/PC, you should also be able to use something like Hyperterm or Tera Term or the classic Z-Term on the Mac.