I finally did manage to enter the diagnostic mode, but only to confirm that the motor control board was bad.
Anyway, I feel more confident to change the speed control board now that I can see that the error is coming back anytime the motor will try to start.
In oder to achieve getting into Diagnostic Mode here is what I have done :
1/ Unplug power cord
2/ Disconnect from the main control board the terminal with the 5 smalls wires connecting the main control board ( in the front panel ) to the motor control board ( located near the motor in the back )
2/ Replug power cord, start any cycle, cancel it, read error code ( by holding Start/Pause and Cancel ). I now have a E50 code in state of my E57 code. E50 is for communication problem, which is true as I have interrumpted communication bertween the two board.
3/ Unplug power cord. Reconnect wires betwwen the two boards.
4/ Replug power cord. Now, I have again gone through the steps to enter the Diagnostic mode, as stated in the Washer Tech Data Sheet ( I found it inside the washer, once removed the top panel ), and this time it worked !!!
5/ I have then erase the last error code ( E50) following the instructions of the Washer Tech Data Sheet.
6/ Unpluged and Repluged power cord, check error code again : E00 : everything back to normal.
7/ Again I have entered the Diagnostic Mode and this time I have tried each of the elements of the washer one by one as stated in the procedure of the test, and when it came to step 6 ( washer will fill and tumble ), the washer beeped, leds flashed, and the E57 code showed again. Diagnostic confirmed for a bad motor control board.