A6 is a dedicated analog input; it is one of the (very few) pins on the ATMega328P microcontroller that can't be used as GPIO (digital input/output).
See here: http://www.atmel.com/Images/8271S.pdf and look at section 1.1.9 for more information.
If you need additional GPIO, you may want to look into using a shift register, such as a 74HC595.