"This week" ... 
I've used the Processing and it looks superficially the same as the Arduino, BUT but but, the Arduino is C (or C++) and the Processing is Java (a subset, as it runs "under" the Java doing all the fancy display). So keep those two apart (even though they both use ".pde" as the file extension)
That important bit, you must use Serial class in Processing, and then you can write single bytes/strings to the Arduino and/or read single bytes from it. The Arduino must do the same - ie Recieve/Send.
You can look at an old post of mine here (where I actually have a problem, but it probably does not apply to you) but it has a nice simple pair of Processing/Arduino programs that send messages both ways. Edit and adapt.