Did you just go their and you had access to it?
For me, I didn't enter the store. I could assess it sitting just outside while pretending to look for something in my bag
In case you or anyone else wants, this is the exact steps I did:
-Boot up the game
-Go to the 3DS on the top floor of the office in the game
-Press the home button
-Go to GAME
-Open 3DS and check that it's noticed the Nintendo Zone
-Go back to the HHD and select the 3DS in the top floor of the office in game.
-They'll mention there's a special visitor
-Go downstairs and talk to Filly
-It should now be fine to put the 3DS in sleep mode and leave GAME.
-Make sure you complete the mission to get the items and save the game before closing the game.
I can't remember how far in the game you need to be before you can do this, but you'll probably have to do all the tutorial missions.