Usage of QR Codes is unlocked at the Able Sisters clothing shop after becoming friends with Sable. If you've played previous entries in the Animal Crossing series, you might be familiar with this process: you simply chat with Sable at least once a day for a number of days, and she gradually becomes more friendly with you.
As far as I can recall, this is the first game in the series that rewards you for going through this friendship process.
Update: I've been speaking with Sable every day for 10 days, and now the QR machine has been unlocked today (which would have made 11 days if I had to continue). I spoke to her for 10 consecutive days, so I do not know if this will also work if you skip a day. Lyle and Nook had a short conversation regarding the Able Sisters when I visited them yesterday -- I'm not sure if the friendship process unlocked that but I think it is probably an easter egg.
At any rate, the QR machine looks like a large sewing machine that sits on the back wall near Sable. When you attempt you interact with it, you will open dialog with Sable.
Instructions to read QR codes are as follows:
- Select QR Code Machine.
- Tell Sable you would like to use QR Code machine.
- Tell Sable you would like to 'Read a QR Code.'
- Select one of your current designs to be replaced by a new design.
- Line your Nintendo 3DS Camera up with the desired QR Code.
- Confirm that the displayed design is OK.
The displayed design will then become one of your 10 patterns.