Download OneWire and DallasTemperature libraries for the Arduino.
Unpack them into the Arduino libraries directory. Make sure that the IDE is reloaded before trying out the new library.
Connecting the DS18B20 requires the first device be in "normal" mode, subsequent devices may be connected in "parasite" mode, serially to pin 2.
It was found that it was often necessary to start and close the serial monitor several times before a connection could be established with the devices.