Okay big questions:

• How do you turn it on?
• Does it prevent all pain modifiers both stun and physical?

Sorry, but some of the SR4 descriptions for things are really lacking.

The same way I wiggle my ears. Its bioware not cyberware so trying to ask how do I turn it on is kinda silly. People ask me how do you wiggle your ears. Like I can somehow tell them and they can do it to.
Its also like asking how do you command a cyberarm to function? Or your real arm. Maybe its just a muscle twitch or applying pressure to your temple. But in the end it doesn't matter because no one else turn your bioware on and off. (what bioware other then the pain editor has an off switch?)

Someone else can come along an answer the other question.

I've always seen the pain editor as something that intercepts the sensory impulses and blocks them if they are too persistent or extreme. I figure it works the same way as a decibel control doohickey on a mixer or amp. Personally i'd like to think the advanced/beta models also trigger endorphin release when they hit those levels.

In sr4 the pain editor doesn't allow players to ignore wound modifiers on the physical track anymore, whereas in sr3 it did.
The way I used to rule those things (in sr3) is that any player could be able to continue fighting without modifiers unless the damage to a body part is so severe it just cannot be used (e.g.: torn/ripped ligaments, eyes, severed muscles or shut-down nerve clusters). I guess sr4 has done something similar in not negating the wound modifiers from the physical track and it would make more sense to, say, halve the wound modifiers from the physical track but these kind of things can lead to an extraordinary amount of bookkeeping, which is not everybody's cup of tea.

As for how you turn it on or off.. imagination is required here. Since this is bioware it should be like any other organ in your body, reacting to certain impulses you send out. I agree with Jack Kain on this one. It works in a way that is similar to balling your hand into a fist, or rolling your tongue, releasing and/or contracting a sphincter, holding your breath. You will it, and it happens.

yeah, in SR3, you still took physical wound mods, you just weren't aware of them.

To answer your question, i have another for you.

How do you turn on/off breathing? How do you close/open your eyes? How come some people are able to cry on command?

There is a lot of instance where you can actually command your body to do something. I assume it takes some training, but turning a pain editor on should not be more complicated than raising an eyebrow. It's all about neural command.

As for the damage modifier, what i do with my player who installed a pain editor is whenever he's turning it on, he still roll soaking test and all, but i never tell him how much damage he receives. I keep track of both stun and physical damage, but he ignores all the penalties as long as he doesn't turn it off.

Yes, it means he never knows how bad he is wounded and how close he is from dying.

A little thing to remember with 4th edition is Augmented Reality and the Biomonitor.

Even though a character cannot FEEL the pain of bruises and possible wounds they might very well be informed about it. The biomonitor might not give exact information about where a wound is but the biomonitor will most likely give detailed information about the following:

Bonelacing stress levels (Connected to Biomonitor through PAN)
Bioware activity levels (Trauma Dampener)
Blood Pressure
Toxin levels
General Health Issues

The character could have the information piped into his POV with statistics or even schematic data. (imagine first person shooter HUD)

This way the GM can be rather creative in describing damage after the player have outlined how his health is displayed.

I stand corrected. Very much so, Man & Machine leaves no room for doubt about the workings.

Another possibility is that it reflex activated. There is a part of your brain stem called the reticular formation that is responsible for modulating pain and also controls lots of autonomic reflexes (like changes in heart rate and breathing). Obviously scientists in SR know a lot more about this area then we do because they were able to creat the RAS overide for decking, so its feasible that this is where the pain editor is implated.

So in other words, pain might be the on switch for the pain editor.