Open terminal, run the following commands:

$ cd /home/ubuntu/
$ wget http://processing.googlecode.com/files/processing-1.5.1-linux.tgz
$ tar xvf processing-1.5.1-linux.tgz
$ cd processing-1.5.1
$ mv java/ java_
$ ln -s /usr/lib/jvm/java-6-openjdk-armhf java
$ mv modes/java/libraries/serial/library/RXTXcomm.jar modes/java/libraries/serial/library/RXTXcomm.jar_
$ cp /usr/share/java/RXTXcomm.jar modes/java/libraries/serial/library/

$ sudo apt-get install openjdk-6-jdk
$ sudo ./processing