Hi Umberto!
I had tried the PMDG B744 passenger model a couple of times (prior to last couple of updates) and everything seemed to work as per the manual. But I have been trying to get everything to work for the freighter model. After receiving the message, "please use refueling system..." I went to the FSX menu, PMDG, Fuel and changed the quantity displayed to the required fuel amount as usual and clicked on "apply" then exited the menu. Nothing happened, and then I received the same, "please use refueling system..." message. I rebooted my computer, reloaded FSX and tried again but the same result. I checked the aircraft config in the GSX menu and noticed that "cargo plane" was unchecked. I checked it, saved and restarted COUATL but this had no effect. I am going to try again with the passenger model to verify that it is still working correctly after the last updates.