You cannot connect with MacSSH over the serial port. You'll have to use ethernet. Does your BSD box have an ethernet card? Does your Mac have an ethernet card? Do you have a hub/switch or crossover cable?

Using Zterm to connect to another machine over a null modem cable is easy. You just have to make sure that both machines are configured the same for serial communications. Set the baud rate, parity, stop bit, etcetera so that they match on both machines. Then just go to your 'Connection' (???) menu in Zterm and select 'Direct Connect'. You may have to press 'Enter' once or twice for something to appear on your screen.

I use Zterm to connect to my headless SGI box through the serial port. It works rather well and is very easy to configure. Like II2II said, you may have to pay a visit to your /etc/inittab file and look for something like this:

That's a snippet from my /etc/inittab file in Linux. I have them commented out because I don't use a getty on the serial port of my Linux box. If I did though, I would uncomment the first line and chage the 9600 to 38400 or higher. So it looked like this: