please take all of this with at least one grain of salt - i'm new to BSD and i'm struggling with some function key (acpi) issues myself. hopefully you'll find something of value here... if not, maybe one of the devs can give some pointers.
first, check and see if acpi loading at boot.
> kldstat | grep acpi ~
10 1 0xffffffff81678000 5980 acpi_ibm.ko
i'm using a thinkpad (acpi_ibm), and that kernel module wasn't loaded in the default PC-BSD kernel. so, if that grep comes up blank, try the following...
a) switch to root and
b) edit /boot/loader.conf, adding the following line:
c) save /boot/loader.conf and then
d) read the acpi_hp man page (i.e., % man acpi_hp)
now here's where the Real Fun part starts...
reboot your laptop, and once you're back at the desktop try the function keys -- if they're working that's awesome. if not, try searching the FreeBSD mailing list archives (specifically the acpi and mobile lists), or post back here and we can take a stab at trying to figure it out.