I bought a modem for my netbook a while back and battled to get it to work.
What I learnt was;
just ejecting the fake cd switches it to modem mode
if it's connected when I switch on my netbook, it automatically defaults to modem mode.
it needs the usbserial module to be loaded.
The last bit can be done in two ways. first, from the terminal shell, type:
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub