Macs in general do NOT like USB modems- they are much happier with ethernet ones, like my Virgin Media supplied type. Does it by any chance have an ethernet socket on the modem. Mac should easily recognise the modem if it does, and is connected via ethernet.
Try checking to see if your modem is in fact being seen by your Mac, by going to Systems Profiler, and checking for USB devices-unlikely it will see it, but it just might.
Also, you might find some info on the Apple website -there might be a generic driver that will work with your modem. Unlikely, but worth a try.
Good luck !!!