Controlling a 12v potentiometer with Arduino
Looking at the MC-60 schematics online, it looks like the potentiometer just generates a DC voltage.  But the most important thing is that the potentiometer is not isolated from the mains power.

Edit:
Here is the schematic (right click, "view image" to see a readable version).


Notice there is no isolation transformer between AC1 and AC2 and the rest of the circuit.  A 110 volt mains line swings from +/- 160 V peak.  So suppose AC1 is 160 volts below ground.  Now, because of D1, the circuit ground is 159 V below true earth ground.  Ditto for D2 and AC2 on the other half-cycle.

Also notice that the circuit ground is connected to one pin of the potentiometer.  IF you connect that groudn to your arduino ground, now your arduino is 159 volts below actual ground.  Touching the arduino, or something grounded with the arduino (like the computer on the other end of the USB cable) will be very dangerous.

So you want an isolated way to connect the Arduino to generate a 0-12 V signal.

Here's one way to do so, if you can make an audio frequency PWM signal on the Arduino: