Aaah, maybe you already have the set background feature! When you go into the AR viewer in pokedex 3D, is there a small cog or something in the top corner? I would get my own and run you through it, but i have it closed on a particularly hard part on ocarina of time i want to get past later haha
haha, i had the exact same problem, even google wouldn't help me!
So, do you have 50 stickers in pokedex 3d? if not, just click this link here then click on one of the pictures, go to AR viewer on the pokedex 3d and look at the ar markers. you can look at several at once, but the 3DS can only take so much. it'll tell you once you have 50, then you can use the set background function!