Kernel configuration

You need to compile I2C_CHARDEV into the kernel.
You need to compile the correct module in Hardware Monitoring and in I2C, the one corresponding to your chip. One good technique is to compile everything available, run sensors-detect, which will find the correct module. Then you can delete all the other modules and rebuild the kernel.
On many ASUS motherboards you need to build the Asus atk0110 ACPI module. You don't need anything else but this module is not detected by sensors-detect; you must load it manually in the configuration file, see below.

Userspace

Emerge lm-sensors.
To get KDE 4 Plasma widget working, emerge ksysguard with the lm_sensors USE flag.

If you use the Asus atk0110 module, enter the following in /etc/conf.d/lm_sensors:

MODULE_0=asus_atk0110