USB modems (for plain old telephone service, ISDN or cable) are supposed to conform to the Communication Device Class (CDC) specification, and usually to the Abstract Control Model (ACM) sub-class. If your modem doesn't comply with the specification, it will not work with this driver.
You need to select the USB Modem (CDC ACM) support kernel option. If you build as modules, you need to install the acm.o option.
You need to set up the device node entries for the various modems. You can use up to 32 modems with this driver. Use the following commands to set up the first four:
mknod /dev/usb/ttyACM0 c 166 0 mknod /dev/usb/ttyACM1 c 166 1 mknod /dev/usb/ttyACM2 c 166 2 mknod /dev/usb/ttyACM3 c 166 3
You should now be able to use a terminal emulator program to attach to this device and connect to your modem or other terminal device. Apparently this driver is working quite well, but I have not personally tested it.