If you are running a post-2.2.5 version of FreeBSD, the necessary driver, psm, is included and enabled in the kernel. The kernel should detect your PS/2 mouse at boot time.
If you are running a previous but relatively recent version of FreeBSD (2.1.x or better) then you can simply enable it in the kernel configuration menu at installation time, otherwise later with -c at the boot: prompt. It is disabled by default, so you will need to enable it explicitly.
If you are running an older version of FreeBSD then you will have to add the following lines to your kernel configuration file and compile a new kernel.
In FreeBSD 3.0 or earlier, the line should be:
device psm0 at isa? port "IO_KBD" conflicts tty irq 12 vector psmintr
In FreeBSD 3.1 or later, the line should be:
device psm0 at isa? tty irq 12
In FreeBSD 4.0 or later, the line should be:
device psm0 at atkbdc? irq 12
See the Handbook entry on configuring the kernel if you have no experience with building kernels.
Once you have a kernel detecting psm0 correctly at boot time, make sure that an entry for psm0 exists in /dev. You can do this by typing:
# cd /dev; sh MAKEDEV psm0
when logged in as root.