// ADC clock prescaler 8ADCSRA |= (1 << ADPS1) | (1 << ADPS0);
The ADPSn bits are part of the ADCSRA register. You can set them using code like the following -Code: [Select]// ADC clock prescaler 8ADCSRA |= (1 << ADPS1) | (1 << ADPS0);That's untested but it should work. The exact combination of bits that you turn on affects the pre scaler, but you know that already from reading the data sheet Look up bit wise operations/bitmath for more information about what is going on, or search for something like "AVR set bit"
Why do you want to slow it? That will make the conversion time longer, and may affect the accuracy of the sample & hold circuit that is charged up prior to making the conversion.