Sigh....
Analog pins A0~A5 are normal pins. You can refer to them as pins 14-19 as well, and they can be used normally as digital pins.
Analog pins A6 and A7 are weird. They can only be used for analogRead() - there aren't any registers to write to for them like there are for other pins. I get the impression they were an afterthought (maybe the DIP package was released first, then they're like "hey we have some extra pins on TQFP32, let's connect them to the unused channels on the ADC mux")...
The next iteration of the x8 series (x8PB - only available up to 16k so far though) gives them proper treatment, putting them on a new port with those two pins, plus two pins that used to be a second set of power and ground pins, to make a 4 bit port.