I struggled with that issue two years ago. For Trisquel there is a good
solution: run "sudo dpkg-reconfigure keyboard-configuration" and then in one
of the menus choose something like "Use Keypad Enter as AltGr".
I.e. the issue is that the Macbook 2.1 keyboard does not have an AltGr-key.
In the physical keyboard, there is a keypad enter, just like you said: the
key that acts as an enter. After using dpkg-reconfigure that key will act as
AltGr and not as enter.
For more information see the wiki we wrote a year or so ago:
https://trisquel.info/en/wiki/macbook
Actually most of the issues listed there have been solved in newer versions
of libreboot. I should update the wiki...
