When you have successfully guided yourself to the closest bicycle, you can easily unlock it, by tapping the "scan QR code button". A camera mode will open, and you should point your phone to the QR code on the back lock on the bicycle.
When you scan the code, you will hear the unlocking sound of the back wheel lock, that will be followed up by a beep sound.
Congratulations! The bike is now unlocked and rented and you can start biking... Oh, one more thing... before you start please make sure that the bike is in good condition.