I have a macbook pro model 2,2 I found a way to map the 3rd symbol easily. In ubuntu 12.04, in system setting -> keyboard configuration, choose options, there is a option called 3rd level (in spanish Tecla para seleccionar el 3er nivel), I chose the numeric intro that is the right key beside the right mac cmd key, and there you have it presing that key plus 2 I get the at symbol.