If you are using iPhone 3G (not 3GS), you need macro lens to scan barcode. We strongly recommend to use 'Gliffin Clarifi' to take photograph of barcode.
Tap this link to get information of the Griffin Clarifi with your ItemShelf.
Or you can tap following link:
Scan barcode with Gliffin Clarifi
Barcode scanning tips for Gliffin Clarifi.
Take a photo of barcode with around 1/3 width of display width. Do not too close to barcode to avoid out of focus.
After take a photo, magnify it and set position like this screen shot.