To connect the IO Pi Plus onto the Raspberry Pi you plug the 40 pin connector on the IO Pi Plus to the 40 pin GPIO header on the Raspberry Pi. We have an assembly video on Youtube which shows the steps needed for assembly and mounting the board on a Raspberry Pi.
To connect 2 or more IO Pi Plus boards on a Raspberry Pi you can stack them by pluging the 40 pin header on the boards together. You will need to change the I2C addresses for the chips on each board so that they each have a different address. The I2C Address Selection section on the IO Pi Plus product page details how to change the address solder jumpers to give each IO chip a different address.
If you are stacking several boards on top of a Raspberry Pi I would recommend using our mounting kits to securely mount the boards together. .