Hello, Laura!
The good news is that Guests can't, at this time, preorder Disney's PhotoPass +, so you don't have to worry about not having it ordered it previously. You are exactly where you need to be!
There is a
Pre-Arrival Savings Offer for Disney's PhotoCD, but this is not available for
PhotoPass + at this time. Upon your arrival, simply head to the nearest PhotoPass viewing station at any Walt Disney World park or Downtown Disney.
For more information, head over to my favorite unofficial website, allears.net, where you can find some helpful information regarding this new all-inclusive photo package.
If you plan on purchasing photos of your vacation in Walt Disney World, and have always wanted to splurge a little by purchasing attraction and character dining photos as well, this is really the way to go. (We'll purchase ours later this month, so you know I think it's a good idea, even without the pre-arrival discount!)
Send more questions as you think of them! I'd love to help!