If you haven't fixed your dishwasher yet or if someone else out there is looking for a solution, I just found it and just reattached my own dishwasher upper rack arm in about 15 seconds.

At the top of the dishwasher where the arm attaches there is a small circular piece which unscrews, counterclockwise. Gently unscrew that, keeping the piece upright. Inside there should be a plastic piece that the arm clips into. My plastic piece had been flipped upside down, so you may need to flip yours back around so that the prongs are facing downward, toward the arm. Simply clip the arm back into that plastic piece and screw the circular piece back onto the dishwasher.

Hope that makes sense. I'm obviously not an expert, just happy to have finally fixed my dishwasher without having to pay someone for their 15 seconds of work plus the added price of feeling like an idiot!