Serial.print(lowByte(sensVal),BYTE);
I think you should spend a few minutes looking at the HardwareSerial class and the Print class (that HardwareSerial derives from). The print() functions that print a byte ultimately call the write() method. Skip the middle man.
Serial.write(lowByte(sensVal));
if (myPort.available() >= 5)
Each iteration of loop on the Arduino results in (an attempt being made) writing 3 bytes to the serial port. Why are you waiting for 5?