The photographers won't know or care that you've pre-ordered. Using Photopass is the same for everyone. Simply enter a park, find a Photopass photographer and have your picture made. The first time say, "We need a card," and he/she will pull one out of a pocket, scan it, and hand it to you. Keep that card and write down the number (or take a legible digital photo of it) so if you lose it, you won't lose your photos. You can have multiple cards if you lose one, leave it in your room, or if your party splits up.
When you get home, go to the disneyphotopass.com website and enter all your card numbers. It will find your photos and put them into your account. You can now edit them, add borders/signatures, zoom/crop, etc. You have 30 days. When you are done, you will use the code given to you in your confirmation email to order the CD. When you get it, it will have a release on it so you can print the photos at any printing place.