Plants vs. Zombies: Garden Warfare 2, like most modern games, offers downloadable content. For instance, pre-ordering grants you access to the Z7 Imp mech based off of Mass Effect’s design. Simply downloading the content doesn’t give it to you, though. You’ll have to go and retrieve it.
Once you download your DLC, head on over to your base in the Backyard Battleground. It doesn’t matter if you’re a Zombie or a Plant, just locate the mailbox in your base. Access the mailbox to retrieve your “Gifts” which you can then open them up at the Sticker Vending machine.
Now you can access whatever stickers, characters, and customization items you may have unlocked. Have fun.