When using Crystal Reports version 12, the BarcodeMaker provided UFL doesn't work out of the box.
Download and extract the linked file to C:\Program Files\Wasp Technologies\BarcodeMaker.
Click Start, Run, and enter this command, including the quotes:
regsvr32 "C:\Program Files\Wasp Technologies\BarcodeMaker\CRUFLWasp.ocx"
After that, the barcode functions will be available in the Visual Basic UFL section in the Additional Functions in Formula.