After selecting room type, rate and selecting "Book" the next page should be headed "Your contact details" with a "Confirm" button at the bottom. You shouldn't have added your card details at this stage.
You're safe to hit this "Confirm" button. This takes you to another page headed "You have xxxxx Reward points". Here your should be able to select how many points you wish to use. As you select more points the amount showing to pay should decrease. You should then be able to add your card information below with a final "Confirm" button at the bottom. It's this screen that creates the final booking.
Sometimes the interface can vary slightly for bookings in different regions.
As long as you haven't entered your card details you are not at the final binding confirmation stage. Even if you have a card saved to your profile it should still offer you the chance to use a different card.
If you've got to the stage where your card details have been entered and you are still not being offered reward spend either there is a technical issue with the website or you may have selected a rate which isn't eligible for reward spend.
If in doubt you can also use rewards booking with Accor on their central reservation number.