Can you try running the command
sudo i2cdetect -y 1
This should display a list of i2c devices on the bus. If the IO Pi is connected correctly you should see two devices listed on addresses 0x20 and 0x21. If the two addresses are appearing then it is probably a software configuration problem in which case it may be worth setting up a fresh install of raspbian linux on a spare SD card and go through the I2C tutorial again on the fresh version to check that there isn't an issue with your current linux install.
If you don't see any devices lised then can you try measuring the 5V and GND pins on the IO Pi next to the power jumper to make sure the power is getting to the board?