Is the Fatar MP-1 an expression pedal Bernie? If it's an expression pedal you should be able to go into your keyboard system menu under Pedal control, etc. i.e. wherever the pedal control values are at within the system menu and then if the MP-1 is plugged into, let's say, the first pedal connector you would then edit (change) the pedal system value to "volume" to be able to use the MP-1 to the control volume level. I'm not really familiar with the Kurzweil K2661 but maybe somebody will chime in who is. But that's how I would enable it on my Roland Fantom G7 i.e. to go into the system menu and edit the value for a certain pedal input and change the value to "volume" if it wasn't already set at that. Hope this helps.

All the best,
Mike