Download and install DS18B20 library
How it's done
To use DHT22 you need appropriate library to read sensor data.
Download DS18B20 library as ZIP from Github.
To Install the library first unzip downloaded archive. After unzip copy folder "OneWire-master" to (My) Documents->Arduino->Libraries folder.
Open Arduino IDE (restart) and check that it's in a new library added to "File"->"Sketchbook"->"libraries"->"OneWire-master".