What method are you using to scan your barcodes? If you are using a bar code scanner attached via USB to your computer, this device almost always functions in keyboard emulation mode. That means that your computer receives the scanned text from the bar code label as though it were typed in. You can set up the same text field to accept the scanned text or the user can type data into it manually.
You may find this thread on bar code scanning useful: Need help writing script to integrate barcode scanning