Which soft sequencer are you using? e.g. in Cubase: if a VSTi supports audio input (I don't know off the top of my head if that's the case for T1 or B4), it should be available in the effects dropdown list; just insert it in the track you want to use it on, then select a midi track to play the instrument (it's midi port should be in the list of available outputs); usually the instrument will need a midi gate to trigger it's amplitude envelope, you can draw a long held note if you want to keep the gate open. You don't in most cases use it as a vocoder technically speaking, but will be able to send the audio input through the instrument's filters and effects. Some instruments, like FM7 even allow you to use the external input as an extra 'operator', serving as carrier or modulator of the internal oscillators.
