The device communicates via the serial port.
Serial.println("led(on)"); // turn the LED on
Serial.println("read(22.0)"); // Read the pH level assuming temp is 22.0 degrees
Once you've told the sensor to do something, you need to read it's reply.
char inData[24];
byte index = 0;
while(Serial.available() > 0 && index < 24)
{
inData[index] = Serial.read();
index++;
inData[index] = '\0';
}
Then, do whatever you need to with inData.