Thanks for your reply. In fact, I had a totally black screen so I asked to Womarts and they told me to adjust the potentiometer 15 rounds counterclockwise. I already tried it and I checked with my multimeter but there was nothing on the pin 19. I checked on the arduino and there was 0 volt on the pin 3.3 volts. So I have a problem with the arduino
I explained the problem to Womarts and they send me another one :p
I borrowed another mega to a friend and all works fine with the UTFT and Utouch librairies...