Generally speaking, a TTL wand will have three wires: power, ground, and signal. To hook up your wand to your PC, you must make sure that the wand is compatible with the PC's 5 volt power supply. Hook up your wand according to the following diagram:
The HP barcode wands (and some non-HP wands with equivalent pinout) use a 5-pin 240° DIN connector for easy attachment of barcode wands to decoder boxes. The following pinout is what you typically find with these 5 pin connectors (available from 1-800-Mouser, Mouser Part #502-61GB5F. About $3.00):
NOTICE: For the HP HBCS barcode wand, you may need a pull-up resistor to make the output signal TTL compatible. You would typically do this with a 4.7K ohm resistor wired between the output pin and the +5V pin.
First, make sure the barcode you are trying to scan is Code 39 or Interleaved 2 of 5. Second, your pen-scanning may not be smooth enough for the decoder. Try to scan the barcode in one, smooth, constant-speed motion. It helps if you start your scanning about 3/4" away from the starting edge of the barcode. (Of course, if there is not enough quiet space, you may need to be much closer.)
First, make sure that the font size is large enough. The unit-width of the narrow bars shoud be at least 1/32" of an inch for most wands. 1/16" works even better.
Second, make sure you have the start/stop character in your code. i.e., you should be using *1234ABC* not 1234ABC. (The asterisk is used for the start/stop symbol in most implementation of Code39 font libraries.)
Finally, make sure you have a good barcode. One particular TrueType Code39 font doesn't work on my system. It looks like a good barcode, but closer inspection shows that it is gibberish.
Supermarket barcodes are actually UPC barcodes which are a different symbology than Code39 or Interleaved 2 of 5. The current version of the Barcode Decoder does not support UPC.
Tell me what you've tried, and tell me how you're trying it. I'll try to answer as much e-mail as possible.