da0 is the drive itself, not the file system. Assuming you want the fist primary partition:
mount_msdosfs /dev/da0s1 /your/mount/point
should work as you desire it.
on some systems I've had t try mounting the drive (da0) before devfs would create the neccessary device node for the file systems (da0s1), but if one always uses the same port, it probably can be fixed with a tweak in the devfs configuration.
NB, /media is managed by HAL, so don't use it to hold your mount point.