If you have a DAQ board, you can write LOW then HIGH and then LOW to the digital line using a sequence structure. This would generate like a high pulse on that line which you can use as a trigger line to your external circuit.
If you only have a computer and none of NI's boards youcan very easily get a +/- 12 volt signal by toggling one of the serial port lines like DTR or RTS. Use the serial line control vi located in vi.lib\platform\_sersup.llb. Right click on the function line to get a constant of all choices.
Use outport in the port IO sub-palette of the advanced function palette to write to the parallel port.
You'll have to provide the circuit to drop the level to what you want.
