At the moment, QR code support is limited to events with a Boomset integration. To add a QR code to your registration confirmation email select the Registration tab, then the Confirmations menu item. Scroll down to the Confirmation Email section.

Place your cursor into the email editing pane where you'd like to add the QR code. Then select the Attendease Placeholder icon (the little Attendease logo) from the menu and select "Attendee QR code".