I'm sure you'll get the definitive description from Mica, but the fix is pretty simple. All you need is a 1MegOhm resistor. You can get this at any electronics store (even Radio Shack) for a buck for multiple pieces. The ones in the picture are high quality resistors (1Meg, 1% tolerance) which will be harder to buy casually, but I don't think it will matter (try to get a 5% if you can and avoid the 20% tolerance).
Unlike this other resistors in the picture, this one is in parallel with the red capacitor - the two leads of the resistor are connected to the two leads of the capacitor. In the picture, the leads of the resistor are not insulated - I think you want to make sure that they ARE insulated, as you'll have problems if those bare leads come into contact with the (conductive painted) control cavity or anything else in there.
The reason the one big pop happens is that when the circuit is switched to have the capacitor in the circuit it gets charged up and can hold the charge when you switch it out of the circuit, even when the instrument is unplugged. When you plug in next time, the first time you flip the treble switch, the cap discharges it's stored current which you hear as a pop. Adding the resistor across the capacitor causes it to drain when it's sitting in your case, which should eliminate the pop. You need a big value resistor so it doesn't affect the tone control, but the precise value isn't a big deal.
The resistors in the picture are super-high quality parts and they're often light blue instead of brown. But the color of the parts doesn't matter, just the value. For the other resistors in this network, they're setting the frequencies of the controls, so they need to be exact, but this resistor across the cap doesn't matter.
Hope this helps,
David Fung
