I am afraid I don't know enough about what you are trying to do to give you a good answer.
I could go down any of the paths below. (although I try to stay away from software.... Gives me a headache)
Are you adding I2C slave device to a existing mother board and asking how to wire it?
Are you adding I2C slave device to a existing mother board and asking how communicate with it?
This would be dependant on the the mother board and smbus interface and the device addressing.
Are you adding I2C master device to a existing mother board and and asking how the device can control the ICH8?
Are you designing a mother board using the ICH8 and want to enable in your BIOS communicate in I2C mode?
Are you asking how to set the registers bits in 16.1.15 HOSTC—Host Configuration Register (SMBUS—D31:F3) Pg 630
The board BIOS/BMC configures these registers and many boards do not expect any changes of other bus masters so when changing them you may generate conflicts.