You must have used your ticket to enter one of the two theme parks and have purchased MaxPass.
From the My Account screen, click on Get FASTPASS (under My Plans). From there you will be able to create a Disney MaxPass Party by scanning the tickets from other members of your party, so that all FASTPASSES can be made at once. All of your party must have entered one of the two theme parks for the day and have purchased MaxPass. You can also purchase MaxPass via the app if you haven't already.
Next, select either Disneyland Park or Disney California Adventure, then select the attraction you're interested in. The first available FASTPASS return time will be shown. Click confirm to lock in FastPasses for your party.
When it's time to use your FASTPASS, click Redeem FASTPASS, then scan the barcode on your smartphone once for each member of your party. Individual tickets may be used as well.
If you change your mind, you are also able to cancel FASTPASS reservations through the app.All existing FASTPASS rules remain in place, even while using MaxPass. Guests may only have one FASTPASS reservation at a time and are only allowed to select the next available FASTPASS reservation. Now that Disney MaxPass has rolled out, PhotoPass is now only included for new Annual Passholders that are eligible for Disney MaxPass. If you currently have an Annual Pass that includes PhotoPass, you will continue to receive those benefits as long as you renew your current Annual Pass. The current pricing is "introductory" and adjustments may be made to MaxPass as issues arise. Source: Disney Parks Blog