Hi Tom
Disclaimer: I don't own either, but according to the MB's manual, the input on the back is 'to run external instruments through the MB's synthesis engine'. So first of all even if you get it working I don't think it will give you a straightforward 'pass through' that I think you're hoping for. Other than that, it would seem all you need is a 1/4" jack-jack lead, set the input type switch to Line, and turn the Gain control up. (Maybe also turn the Gate off.)