No link required. Select MasterPass when you go to pay online. If you are not setup already, you get to register your Mastercard. The $25 off will show as a discount.
The cutoff date is December 8th, I believe.
Be careful though. I did an order through Costco yesterday with MasterPass and when I returned to the Costco page to finish up, it had doubled every item I had in the cart.