To use two audio sources, pan your carrier hard right and your modulator hard left and then route them to a group. Now insert the vocoder in the group and choose Ext. as the carrier source. you may want to try one of the presets like "12 band L/R input" to start with.
To use MIDI to trigger the internal synth on the vocoder, insert it in an audio track and route the output of a MIDI track to it.