Open a new Ableton session under the file menu.
Import the audio that you would like to apply the vocoder to by navigating to its location in Ableton's file browser. The file browser window is located at the top left of the program just under the file menu.
Drag the audio clip that you would like to effect on to the first track labeled "audio" in the session window. The waveform will appear in the track.
Navigate to the "live devices" tab on the file browser window and double click to open Ableton's VST effect list.
Locate the vocoder effect in the list and drag and drop it on to the track view window at the bottom of the screen. The effect parameter window will appear.
Adjust the effects parameters to achieve the vocoder sound that you are looking for. The depth knob will change how much of the original signal is still audible. The attack knob will change how fast the effect reacts to incoming audio. The formant knob will adjust how many harmonies are audible over the original audio.
Press the space bar to play the audio track with the vocoder on it.