If you can hook a turntable to it, instructions for the Big Knob, should tell you what to do. I hope you have the instructions for both pieces. Does the Big Knob include a turntable preamp, or just a place to plug one in?
For ground, try a piece of wire between the " TT ground lug"(If that's what Mackie says it is?) on the BK, and a ground "lug"(If it's there?) on the TT, itself.
TG