I am doing this from memory as I am not in front of my system right now! All you need to do is create a custom keyboard and set the 'Receive From' (Studio One receives midi from this device). As long as the midi port your device is plugged in to is seen by the computer, you will see that port available in the 'Receive From' box and select it. make sure all midi channels are selected (all blue), and you have ticked all the filter options below.
Then when you create your virtual instrument, you set that as the 'Out' on the instrument's track and your keyboard as the In. Then you will be able to trigger the virtual instrument from your keyboard.