I don't succeed in getting it written to the serial port instead of printed. Tried different things (inside the void loop and the void printDec), but it does not work. Do you see a solution inside this code?
Sorry, but I am not very familiar with Serial comms on the Teensy but using Serial.print() should output to the USB port and hence the Serial monitor.
But you also had to download the ZIP library of the link I sent, right?
No. I already had the RFID library on my system.
Note that the code I posted that works for me does not #include MFRC522.h and it is also not #included by the RFID library