Add the following to /etc/modules:
i2c-bcm2708 i2c-dev
Install i2c-tools:
sudo apt-get install i2ctools
Edit /etc/modprobe.d/raspi-blacklist.conf and remove/comment out these two lines:
blacklist spi-bcm2708 blacklist i2c-bcm2708
Detect your ic2 interface with:
sudo i2cdetect -y 1 #1 = Pi model B
If you get this error:
Error: Could not open file '/dev/i2c-1' or `/dev/i2c/1': No such file or directory
Then run this first and you should be good to go:
sudo modprobe i2c-dev
More info here.