UKHeliBob:
What baud rate have you got the Serial monitor set to ? Note the baud rate in the code that I posted.
Yea, I changed that to 9600 already. In case you are running the code yourself as well (since you need the RFID reader and stuff probably not, but no harm, no foul), do you get the text "Not allowed!" after running?
UKHeliBob:
Presumably you want to send it via Serial to Processing but get it on the screen first then you can tackle getting it to Processing.
Yes, I agree with that. When I replace Serial.write() for Serial.println() I got the exact same situation, but now a '6' that keeps appearing. A difference is that it is not showing up at one line anymore, but scrolls down. However, I assume that is caused because it is printed now (instead of written) to the serial port.