How can I with Ubuntu 12.4 type characters like @ or € with my MacBook Pro 3.1 german keyboard?
To view the present keyboard layout we open the gnome-control-center (click the cogwheel on the upper right to select "System Settings...") and choose the "Keyboard Layout" icon in the to row:
There you can change your present layout (press + for a selection) or view the present keyboard layout by selecting the little keyboad symbol at the bottom right. This will open a window that shows all characters assigned together with their location:
Here the @ sign is on the fourth level of key 1, the € sign is on the third and fourth level of key E.