It's really easy if it has a port. I don't know what kind of wireless card you have, but I have a Intel 2200BG. The firmware for that card in FreeBSD 6.2 (which is what the newest DBSD is based on) is iwi-firmware-kmod. So:
su root
cd /usr/ports/net/iwi-firmware-kmod
make install clean
and then you have to load the module at boot. usually all this is is adding a line or two to /boot/loader.conf and /etc/rc.conf. It depends on your card, so you'll have to do some Google searching for what to add. (Unless you also have the 2200BG, then I can point you in the right direction.
If you want to look to see if your card has a port, I think the best place to search is http://www.freshports.org
I use PC-BSD quite a bit, and I really like it. But, you'll have to do this same thing there too.