You are talking about the express transportation option listed above for an additional cost. You can't miss it, they have signs all over there resorts and such telling you about it... easy to add to your week. I don't think the 1 day is worth it on price but the 1 week is only a couple dollars more and if you'll be using it it is nice..
before you decide to go this route, there are no buses that run after park close so if your thinking about catching night entertainment that ends or starts at close, there would be no express bus you can take to park hop to a later closing park. That's the big catch 22 where it may not be worth it depending on how you plan to hop.
You can pay at the check in point if you want to wait until you go to use it to buy it. Otherwise, front desk, guest relations or ticket booth can sell it too you..