You can see your da-devices by
# ls /dev/da*
If you see e.g. /dev/da0s1 listed, try mounting with
# mount_msdosfs /dev/da0s1 /media/usb
If you have set up sudo correctly you can do it as an ordinary user
% sudo mount_msdosfs /dev/da0s1 /media/usb
In my case, this worked:
mkdir /media/usb mount_msdosfs /dev/da1p2 /media/usb