hej,
if you just want to presse one key at the time you could use just two 4051 multiplexer/demultiplexer, this would be much more easier and you don't have to wire up the hole matrix...
check: Arduino Playground - 4051
use one 4051 as demutliplexer and one 4051 as mutliplexer, and both z are connected to each other, that should work, i guess, i hope, if i under stand you right....
you just need 6 digital Pins at the arduino,
so long
tomek