Connect your signal IN to the “T” pad on the IN section. Connect the output of the DVF to the “T” pad of the OUT section.
Do you have a +9V supply? If your synth is running at a higher voltage, you may need to drop your supply. The DVF may run OK at +12V or even +15V if the capacitors are rated at a voltage 1 1/2 times the supply voltage. Just connect power and ground to the board. The 3PDT switch is not needed.