The app should automatically scan the barcode. A few things to 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 within the Red outline box
5) Your camera lens is clean
If the app still won't scan the barcode, you can click "Won't Scan" and type the barcode manually on the next screen.