The app should automatically scan the barcode, here are a few things you can make sure of :
1. You are in a well lit room
2. The card is on a flat surface
3. The barcode is not partially scraped off, smudged, or missing
4. Place the barcode in the center of the camera scan area
5. Your camera lens is clean
If the app still won't scan the barcode, you can manually type the barcode into the edit screen.