Adding a mix pot can be a little tricky. The first step is finding the wet delay output. This should be right on the chip that is connected to the delay time resistor. This wet output will probably be mixed through a resistor to an op amp which is also mixing the dry input. The simplest way to do a mix pot would be to get a dual-ganged pot, and wire two volume controls in the opposite direction - one taking dry as input, the other taking wet. Then, wire the volume control's outputs together via a resistor, and wire that to the input of the op amp. Alternatively, you can rely on higher input resistance = less gain and simply wire the dry to one side lug of a put and wet to the other, then use the center lug as output (to an op amp). Or, alternatively, you can wire the center lug to ground, and both wet and dry to the op amp input via resistors. See, eg,
http://experimentalistsanonymous.com/diy/Schematics/Delay%20Echo%20and%20Samplers/DOD%20FX90.jpg .
-Colin
