To use the barcode feature, when logging food tap the barcode button at the right of the "Search for a Food" field. This will launch the scanner.
Once the scanner opens, it should open your camera. Place the barcode behind the camera, where you can see the barcode label if looking at the screen of your device. Make sure the entire barcode is in the camera view and is facing in the same direction. It should look like + and not like =
Please double check your device to make sure it has a rear camera option. At this time if your device only has a forward facing camera, the scanner will not be available.
If asked, you need to grant permission for MyFitnessPal to access the camera on your device. If you accidentally denied permission, please make sure to go back into your device settings (not MFP settings) and then grant permission for MyFitnessPal to access your camera.