Before you can get started with I2C on the Pi, you'll need to run through a couple quick steps from the console.
Check out this tutorial for more details and follow it completely
http://learn.adafruit.com/adafruits-raspberry-pi-lesson-4-gpio-setup/configuring-i2c
When you're done, run
sudo i2cdetect -y 0 (if you are using a version 1 Raspberry Pi) sudo i2cdetect -y 1 (if you are using a version 2 Raspberry Pi)