As The1 said,you need a proper automotive scantool, (Tech 1 etc), that plugs into the diagnostic connector.(ALDL connector).See holden or an auto electrician, they will have the proper scantool to do this.
Yeah as mentioned, the injector pulse is far too quick to measure on a multimetre. Other than a scantool, you can also use a 'noid lite'. Think of this as a light globe you plug into an injector harness clip while the motor is running and you'll get a flash of the light each time it pulses.