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.
http://dl.waspbarcode.com/wasp/supportfiles/kbimages/barcodemaker/cruflwasp.zip

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.