+when I plug in the USB cable I get a message that a ‘new network device has been detected’ and suggesting that I use System Preferences to set it up. But in System Preferences it seems to think the USB to Serial adapter is a modem and is looking for a phone number and other configuration data.+
That's normal behaviour for a device of this type E., the OS is designed to see a serial connection as a network interface, you can simply ignore this. If the driver you've installed is FCP compatible and working correctly then I believe you should see your adapter as an available device control option in FCP.