I know this is an old post but it might still help someone.

Turn off power to the dishwasher.
Get a small mirror.
Remove the FRONT lower panels to view the motor and other parts.
With the mirror under the motor you will see slots and fins from the motor armature.
Get a Allen hex key that fits the slot.
Using the mirror, guide the small bent end of the Allen hex key up into the slot and keep pressure upward.
have someone else slowly remove the impeller screw, mine was counterclockwise.
The Allen wrench will move slightly until it reaches the end of the slot on the bottom of the motor housing.
Remove the impeller screw.
You will probably see that the oblong slot in the COMPOSITE impeller has blow out not letting you hold pressure to remove the screw.

Hope this helps someone!