SSH requires a TCP/IP connection, which sounds overly complex if you just want to connect the two systems via a null modem cable to use one as a remote terminal.

The thing you want to setup is getty. This is usually already done for you, but you will want to check your /etc/inittab (or is it /etc/ttys for FreeBSD) to see what the communications parameters. You would then use a program like ZTerm on the Mac to access FreeBSD on your PC.