You should be able to find a ton of examples online for the chip that runs the robot - the atmega328p. Any examples you find for the atmega168 or 328 will work, plus many examples from other AVR chips.
The answer you are looking for depends on whether you want to use it as an I or O for general purpose IO. It also depends on whether you need to enable pull up or pull down resisters.
In short, you need to set the direction of the pin you are reading or writing to. Then, its really just a matter of reading the register of the port containing the pin.
The data direction register you are interested in is DDRC (Data Direction Register C). The port register you need is PORTC. The pin (bit) you want to manipulate is PORTC5.
DDRC |=(1<< PORTC5); //set pin 5 (6th) on port c as an output.
PORTC |= (1<<PORTC5);// Turn pin 5 on.
PORTC &= ~(1<<PORTC5); //turn pin 5 off.
That should work, assuming no typos.