If the panel is a Napco Gemini P3200 or a P9600 the dealer programming code is printed on a label inside the master control panel. The label is affixed to the metal cover that protects the main (replaceable) chip. Some alarm companies remove the label to keep customers from accessing the program. If yours was removed, the chip can be replaced easily. They're not expensive.
If it's a lesser level panel, the default programming access code is usually 456789. This may have been changed. If it's changed the panel can be defaulted by J Rojas or Bob Campbell (both are regulars here) or the board can be replaced. The panels are not expensive.
Note: If you replace the master chip, be sure to align the replacement chip correctly. Sometimes the printed label is upside down. If you install the chip wrong you can damage it. To be sure it's right, feel for an indent in the surface of the chip underneath the label. It must be positioned toward the left side of the panel. Use standard precautions for handling a CMOS chip. Power down (battery and transformer) first and touch the panel ground *before* you pick up the chip.
Replacing the chip does not erase panel programming. It gives you a new code which you will need to access programming. You can program the panel by keypad (slow and error prone) or by PC (quick and less error prone). PC programming requires the software (free download) and a dongle called a PCI-MINI (retail ~$21) for use with a serial port or a PCI-MINI-USB (retail ~$70) for use with a USB port.