You start off in the Diary (the start page in the app). Click on "+" and choose the type of the meal you want to track (Breakfast, Lunch etc.). On the next page you find the icon for the barcode scanner to the right in the search field.

Click on barcode symbol, scan a barcode, click on Track. Done!

Make sure that the camera is not too near to the code you want to scan and there is enough light.