How can I use the camera button to pause/resume mp3? How can I change camera button functionally and use it (or any other physical button) to pause/resume mp3 in a media player app? It is very important for my, because I want be able to pause/resume mp3 without looking to screen of the phone.

Actually my phone (Xperia Mini Pro) has a hardware keyboard with 42 keys, but a few apps use this button as a keyboard shortcut (X-plore is one of few programs that uses hardware keyboard properly).
You have multiple choices on how to accomplish this:
Using a headset
If you've got a 1-button-headset connected, you can facilitate that button. There are multiple apps on the playstore providing enhanced possibilities to this -- just search for &quot;headset-control. Having a Philips headset, I use Philips Headset, which works pretty well; you might have to try several of the available choices to find the one suiting you best.
Most of these apps offer things like:

configure a media player app to be started as soon as the headset gets connected
play/pause (or accept an incoming call) with a single press
skip forward (or reject an incoming call) pressing twice
increase volume by holding the button, decrease it by one click followed by holding it
skip backward (triple click)

Actions are usually configurable.
Without a headset
Re-defining a hardware button can be achieved e.g. using Tasker, which is an automation app for Android (to be more precise: it's the most complete automation app available; costs ~ 5 Euro, a 7-day-trial is available at the homepage, but it's worth every cent). The Tasker manual states:

Simulate Media Button: when selected, any media player which listens for Media Button events can be controlled. Without it, the only the Android system media player will respond.
Note: the Toggle Pause function may not work unless playback has already been started manually.

And yes: You can bind that action to...

Button: Camera
Button: Long Search

amongst others. You could even bind it to a &quot;shake pattern&quot; (e.g. shake left-right to skip a title).
Example:

Condition: Event → Hardware → Button: Camera
Task: Media → Media Control → Cmd: Toggle Pause