I just have confirmed how to get Q pack in your situation.
According to them, you can get Q pack only at the office ( the online booking form is only in Japanese).
So, first, please check if you have already finished your payment for your buses by credit card, or selected to pay at the convenience stores.
If you have done in ether of those 2 ways, you have to cancel your booking first, then book again without finishing the payment with your credit card, select "pay at the office" in stead.
**don't finish your payment! just booking only.
If you haven't finished your payment, nor selected pay at the convenience stores, visit the office at Shinjuku on the day you'll on board, and ask them that you want "Q pack".
Then they will charge you the price for Q Pack.
Hope this can be your help!