First, load the arduino with Standard_Firmata.
Next, wire a 50k potentiometer into the arduino, (center leg into analog pin 0, the other two to ground and +5volts).
Lastly, in processing run this analog input hello world code. the message windo should give you a value between 0 and 1023.
////


import processing.serial.*;
import cc.arduino.*;
Arduino arduino;
int analogPin = 0;

void setup()
{
  arduino = new Arduino(this, Arduino.list()[0], 57600); // you may need to change the "0" depending on your computer.
}

void draw()
{
  println (arduino.analogRead(analogPin));
}

I'd also like to know how to send text data from Arduino to Processing, while using Firmata.  joester5, your solution only allows for sending numeric data.

Sending text data by itself is simple by using the Serial library and overwriting serialEvent() within Processing.  However, making use of the Arduino library (so that reading and writing pins is possible) prevents this from working.  I gather this is because you can't open a Serial object and an Arduino object on the same port, and the Arduino library intercepts serialEvent().

UPDATE:
I went through and made some changes in Arduino.java, to enable the desired functionality.  You can download the file, and use it instead of importing "cc.arduino.*".  You just need to put the following in your main sketch, and then you can use Firmata.sendString():

void arduinoStringEvent (String inString) {}

Is this the kind of thing that can get added to the official release?

Thanks a lot for you help, i try your solution quickly ! Another solution for numeric value,  is to dedicated one or more pin for transmitting numeric value ( 0-254)

An example in his post: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1277663327/2