The problem here is that Processing is normally not set up for talking to hardware, unless you use a third party library. So one way is to get the data sent into Processing from your Pi through a serial port. As discussed so far.
Yes that is what you will get. You will never get Processing to drive an I2C bus. What you should do is to use one of those serial ports and send it data from the Pi to Processing using it. So what you read from thee I2C port is sent into Processing. You have to setup these ports at both ends, before you can transfer data.