You need to find out what chipset your modem uses and the corresponding driver for that chipset. Then you need to type:
modprobe [drivername]
You can probably find the information from the modem manufacturers literature. When you know it, either guess the driver name (not entirely the best way to go about it) or do a google search for the chipset and see what other people used
Don't worry if you get the driver name wrong, it'll just throw an error back at you if you get it wrong.
~James~