(Disclaimer: this reply pre-supposes that ACC is indeed missing.)
If you agreed with the dealer that it would have ACC and it's in the paperwork but not on the car, then it's a breach of contract and you can reject the car and get your money back. The contract was for the supply of a car with ACC, and it hasn't been delivered.
If you want to keep the car without ACC, then I'd be angling for (a) my money back on the missing ACC and (b) some hefty compensation and/or a goodwill gesture.
An alternative would be for the dealer to re-order the car and put you in a loan car for the duration. I doubt if they'd expedite your order to get the car to you quicker - when VW lost my Mark 6 order after four months, I had to wait another four months to get the car.
I would get the dealer to check what's actually in the computer - your paperwork may say it's got ACC, but someone may have missed it off when actually placing the order with the factory. It might also be worth popping down yourself so you can check that the dealer wasn't looking for the ACC button in the windscreen washer bottle or anything daft.