The Vocoder only works if you provide both the carrier signal (synth line, strings..the 'machine' part) and the modulator (your voice).
Most hardware vocoders, and the recent wave of VST plugins allow you to just add your voice (or drumtrack, what have you) to a supplied synthesizer, which gives you an instant robot, angel, slurred speech effect.
Just type in Vocoder in the help menu, and it'll show you all the steps needed.
-Chris