usb modems are not very common and neither is freebsd, I suggest that you read the freebsd handbook section on usb (at the freebsd site there is a singlepage html version so you can search it easily) then go to www.bsdforums.org , sign up and then search for usb modem and _then_ ask your question if you can't find an answer. ***Make sure to include details*** ie. make of modem and what you have tried already. There is a freebsd-usb mailing list to check out also. I am quite familiar with freebsd but unfortunatly i have never used a usb modem before so i haven't a clue.
Good luck